Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets carried at fair value
Investment securities available for sale	$ 323,399	$ 1,760,812
Interest rate swaps
Liabilities carried at fair value
Derivative labilities		(29,000)
Variation margin		44,800
Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	901,899
Agency CMBS
Assets carried at fair value
Investment securities available for sale	0	20,277
Fair Value, Measurements, Recurring
Assets carried at fair value
Investments in unconsolidated entities	218,706	83,882
Preferred equity and mezzanine loan investments	183,154	0
Total	3,828,346	22,681,286
Liabilities carried at fair value
Total	1,077,980	17,777,280
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	15,878
Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans, at fair value	1,290,005	1,328,886
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	922,877
Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale	0	50,958
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	375,765	715,314
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	182,925	267,777
Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale	45,007	49,214
Fair Value, Measurements, Recurring \| Residential loans
Assets carried at fair value
Residential loans, at fair value	1,177,298	1,429,754
Fair Value, Measurements, Recurring \| Residential loans held in securitization trusts
Assets carried at fair value
Residential loans, at fair value	355,486	0
Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	17,816,746
Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	0	16,724,451
Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	1,077,980	1,052,829
Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investments in unconsolidated entities	0
Total	0	0
Liabilities carried at fair value
Total	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	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 1 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 1 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Residential loans
Assets carried at fair value
Residential loans, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Residential loans held in securitization trusts
Assets carried at fair value
Residential loans, at fair value	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	0	0
Level 2
Assets carried at fair value
Investment securities available for sale	603,697	2,006,140
Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investments in unconsolidated entities	0
Total	603,697	2,022,018
Liabilities carried at fair value
Total	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	0	922,877
Level 2 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale	0	50,958
Level 2 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	375,765	715,314
Level 2 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	182,925	267,777
Level 2 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale	45,007	49,214
Level 2 \| Fair Value, Measurements, Recurring \| Residential loans
Assets carried at fair value
Residential loans, at fair value	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Residential loans held in securitization trusts
Assets carried at fair value
Residential loans, at fair value	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	0	0
Level 3
Assets carried at fair value
Investments in unconsolidated entities	218,706	191,359
Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investments in unconsolidated entities	218,706	83,882
Preferred equity and mezzanine loan investments	183,154	0
Total	3,224,649	20,659,268
Liabilities carried at fair value
Total	1,077,980	17,777,280
Level 3 \| Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Consolidated SLST \| VIE, Primary Beneficiary
Assets carried at fair value
Residential loans, at fair value	1,290,005	1,328,886
Level 3 \| Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 3 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	0	0
Level 3 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale	0	0
Level 3 \| Fair Value, Measurements, Recurring \| Residential loans
Assets carried at fair value
Residential loans, at fair value		1,429,754
Level 3 \| Fair Value, Measurements, Recurring \| Residential loans held in securitization trusts
Assets carried at fair value
Residential loans, at fair value	355,486	0
Level 3 \| Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	17,816,746
Level 3 \| Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	0	16,724,451
Level 3 \| Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value
Liabilities carried at fair value
Collateralized debt obligations, at fair value	$ 1,077,980	$ 1,052,829

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Financial Instrument Owned, Loans And Preferred Equity

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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