Annual report pursuant to Section 13 and 15(d)

REAL ESTATE RELATED AND OTHER LOANS, RESIDENTIAL MORTGAGE LOANS AND SUBPRIME MORTGAGE LOANS (Narrative) (Details)

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REAL ESTATE RELATED AND OTHER LOANS, RESIDENTIAL MORTGAGE LOANS AND SUBPRIME MORTGAGE LOANS (Narrative) (Details)
 1 Months Ended 3 Months Ended 12 Months Ended 24 Months Ended
Dec. 29, 2016
USD ($)
Sep. 30, 2016
USD ($)
Apr. 30, 2016
USD ($)
Aug. 31, 2015
USD ($)
loan
Jun. 30, 2015
USD ($)
Dec. 31, 2016
USD ($)
loan
Sep. 30, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
loan
Sep. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2016
USD ($)
loan
Dec. 31, 2015
USD ($)
loan
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Federal income tax basis           $ 75,500,000               $ 75,500,000            
Proceeds from sale of investments                           2,777,808,000 $ 1,425,480,000 $ 798,580,000        
Repayment of debt       $ 71,300,000                     11,500,000   $ 35,900,000 $ 39,300,000 $ 188,900,000 $ 787,800,000
Proceeds from notes receivable   $ 109,900,000                                    
Interest income                           91,291,000 95,891,000 127,627,000        
Gain on sale of call option $ 100,000                                      
Gain on sale of call option (amount less than) $ 100,000         $ (1,451,000) $ 19,677,000 $ 8,518,000 $ 89,955,000 $ 7,711,000 $ 23,832,000 $ 1,085,000 $ 10,866,000 $ 116,699,000 43,494,000 52,474,000        
Subprime                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Original number of loans (approximate) | loan           2               2            
Annual servicing fee, as a percentage of unpaid principal balances (as percent)           0.50%               0.50%            
Mezzanine Note Payable                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Face amount of securities sold     $ 19,400,000.0                                  
Repayment of debt     11,700,000                                  
Real Estate Related and other Loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Face amount of real estate related loans on non-accrual status           $ 77,200,000       63,500,000       $ 77,200,000 63,500,000          
Loan Count | loan           6               6            
Outstanding face amount           $ 138,148,000               $ 138,148,000            
Average carrying value of receivables           $ 116,600,000       $ 172,800,000       116,600,000 172,800,000 270,100,000        
Interest income                           $ 40,100,000 $ 36,800,000 49,300,000        
Real Estate Related and other Loans | Corporate Loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Loan Count | loan           2               2            
Outstanding face amount           $ 45,700,000               $ 45,700,000            
Proceeds from notes receivable   $ 109,900,000                                    
Financing receivable           $ 61,000,000               $ 61,000,000            
Real Estate Related and other Loans | Mezzanine Loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Loan Count | loan           2               2            
Outstanding face amount           $ 17,800,000               $ 17,800,000            
Real Estate Related and other Loans | Held-for-sale                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Large loan reporting, percentage threshold (as percent)           3.00%               3.00%            
Loan Count | loan           6       7       6 7          
Outstanding face amount           $ 138,148,000               $ 138,148,000            
Accretion of loan discount and other amortization                           10,540,000 $ 3,203,000 8,867,000        
Real Estate Related and other Loans | Held-for-sale | Corporate Loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Large Loans reporting in the other category - Bank Loans Face Amount           $ 59,400,000               $ 59,400,000            
Loan Count | loan           4       4       4 4          
Outstanding face amount           $ 120,381,000               $ 120,381,000            
Real Estate Related and other Loans | Held-for-sale | Mezzanine Loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Large Loans reporting in the other category - Mezzanine Loans Face Amount           $ 17,800,000               $ 17,800,000            
Loan Count | loan           2       3       2 3          
Face amount of securities sold     19,400,000.0                                  
Outstanding face amount           $ 17,767,000               $ 17,767,000            
Real Estate Related and other Loans | Held-for-sale | Mezzanine Loans | Mezzanine Note Payable                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Repayment of debt     $ 11,700,000                                  
Real Estate Related and other Loans | Held-for-sale | Others                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Large loan reporting, dollar threshold           $ 1,700,000               $ 1,700,000            
Loan Count | loan           5               5            
Outstanding face amount           $ 77,151,000               $ 77,151,000            
Commercial Loans | Commercial Real Estate Loans - CDO VIII                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Face amount of securities sold         $ 12,000,000.0                              
Average price percentage - sold (as percent)         100.01%                              
Proceeds from sale of investments         $ 12,000,000                              
Gain (Loss)         900,000                              
Commercial Loans | Commercial Real Estate Loans - CDO IX                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Face amount of securities sold         $ 45,700,000.0                              
Average price percentage - sold (as percent)         95.35%                              
Proceeds from sale of investments         $ 43,500,000                              
Gain (Loss)         $ 600,000                              
Residential Loans | Residential mortgage loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Number of loans - charge-offs, sold | loan       2                                
Outstanding face amount       $ 3,300,000   771,000               771,000            
Proceeds from sale of mortgage loans held-for-sale       $ 2,900,000                                
Average carrying value of receivables           $ 400,000       $ 2,400,000       400,000 $ 2,400,000 90,500,000        
Interest income                           100,000 100,000 8,300,000        
Accretion of loan discount and other amortization                           $ 0 $ 0 115,000        
Residential Loans | Held-for-sale                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Loan Count | loan           3       4       3 4          
Outstanding face amount           $ 771,000               $ 771,000            
Residential Loans | Held-for-sale | Residential mortgage loans                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Accretion of loan discount and other amortization                           0 $ 0 $ 0        
Subprime Mortgage Loans Subject to Call Option | Subprime                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Outstanding face amount           $ 0               $ 0            
Subprime Mortgage Loans Subject to Call Option | Subprime Portfolio I | Subprime                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Percentage of aggregate principal balance (as percent) for redemption of option           20.00%               20.00%            
Weighted average coupon rate (as percent)           9.24%               9.24%            
Subprime Mortgage Loans Subject to Call Option | Subprime Portfolio II | Subprime                                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                                        
Percentage of aggregate principal balance (as percent) for redemption of option           10.00%               10.00%            
Weighted average coupon rate (as percent)           8.68%               8.68%