Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets carried at fair value
Preferred equity and mezzanine loan investments		$ 180,045
Investment securities available for sale, at fair value	$ 311,284	1,760,812
Interest rate swaps
Liabilities carried at fair value
Derivative in liability position		(29,000)
Variation margin		44,800
Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	901,899
Agency CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	20,277
Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	265,817	591,499
Fair Value, Measurements, Recurring
Assets carried at fair value
Preferred equity and mezzanine loan investments	163,593	0
Equity investments	259,095	83,882
Total	4,196,580	22,681,286
Liabilities carried at fair value
Total	1,054,335	17,777,280
Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans:	1,266,785	1,328,886
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	15,878
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	139,395	922,877
Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	50,958
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	186,440	267,777
Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale, at fair value	43,225	49,214
Fair Value, Measurements, Recurring \| Consolidated K-Series
Assets carried at fair value
Loans held in securitization trusts	0	17,816,746
Fair Value, Measurements, Recurring \| Residential Mortgage Loans Held In Securitization Trust, At Fair Value
Assets carried at fair value
Residential loans:	691,451	0
Fair Value, Measurements, Recurring \| Multi Family Collateralized Debt Obligations \| Consolidated K-Series \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	16,724,451
Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value \| Consolidated SLST \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	1,054,335	1,052,829
Fair Value, Measurements, Recurring \| Residential Mortgage Loans
Assets carried at fair value
Residential loans:	1,090,930	1,429,754
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	355,666	715,314
Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Preferred equity and mezzanine loan investments	0	0
Equity investments	0	0
Total	0	0
Liabilities carried at fair value
Total	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans:	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Consolidated K-Series
Assets carried at fair value
Loans held in securitization trusts	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans Held In Securitization Trust, At Fair Value
Assets carried at fair value
Residential loans:	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Multi Family Collateralized Debt Obligations \| Consolidated K-Series \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value \| Consolidated SLST \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans
Assets carried at fair value
Residential loans:	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 2
Assets carried at fair value
Investment securities available for sale, at fair value	724,726	2,006,140
Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Preferred equity and mezzanine loan investments	0	0
Equity investments	0	0
Total	724,726	2,022,018
Liabilities carried at fair value
Total	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans:	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	15,878
Level 2 \| Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	139,395	922,877
Level 2 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	50,958
Level 2 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	186,440	267,777
Level 2 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale, at fair value	43,225	49,214
Level 2 \| Fair Value, Measurements, Recurring \| Consolidated K-Series
Assets carried at fair value
Loans held in securitization trusts	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans Held In Securitization Trust, At Fair Value
Assets carried at fair value
Residential loans:	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Multi Family Collateralized Debt Obligations \| Consolidated K-Series \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value \| Consolidated SLST \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans
Assets carried at fair value
Residential loans:	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	355,666	715,314
Level 3
Assets carried at fair value
Equity investments	259,095	191,359
Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Preferred equity and mezzanine loan investments	163,593	0
Equity investments	259,095	83,882
Total	3,471,854	20,659,268
Liabilities carried at fair value
Total	1,054,335	17,777,280
Level 3 \| Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans:	1,266,785	1,328,886
Liabilities carried at fair value
Multi-family collateralized debt obligations	1,054,335	1,052,829
Level 3 \| Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 3 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 3 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale, at fair value	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Consolidated K-Series
Assets carried at fair value
Loans held in securitization trusts	0	17,816,746
Level 3 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans Held In Securitization Trust, At Fair Value
Assets carried at fair value
Residential loans:	691,451	0
Level 3 \| Fair Value, Measurements, Recurring \| Multi Family Collateralized Debt Obligations \| Consolidated K-Series \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	16,724,451
Level 3 \| Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value \| Consolidated SLST \| VIE, Primary Beneficiary
Liabilities carried at fair value
Multi-family collateralized debt obligations		1,052,829
Level 3 \| Fair Value, Measurements, Recurring \| Residential Mortgage Loans
Assets carried at fair value
Residential loans:	1,090,930	1,429,754
Level 3 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale, at fair value	$ 0	$ 0

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