Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets carried at fair value
Investment securities, available for sale, at fair value	$ 2,006,140	$ 1,512,252
Interest rate swaps
Assets carried at fair value
Derivative assets		1,800
Liabilities carried at fair value
Variation margin	44,800	8,500
Derivative in liability position	(29,000)	8,500
Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	922,877	1,037,730
Agency CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	50,958	0
ABS
Assets carried at fair value
Investment securities, available for sale, at fair value	49,214	0
Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	1,429,754	737,523
Investments in unconsolidated entities	83,882	32,994
Total	22,681,286	13,972,879
Liabilities carried at fair value
Total	17,777,280	11,022,248
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	15,878	10,263
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	922,877	1,037,730
Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	50,958	0
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	715,314	214,037
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	267,777	260,485
Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities, available for sale, at fair value	49,214	0
Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts, at fair value
Assets carried at fair value
Loans held in securitization trusts	17,816,746	11,679,847
Fair Value, Measurements, Recurring \| Residential mortgage loans held in securitization trust, at fair value
Assets carried at fair value
Loans held in securitization trusts	1,328,886	0
Fair Value, Measurements, Recurring \| Multi-Family CDOs
Liabilities carried at fair value
Multi-family collateralized debt obligations	16,724,451	11,022,248
Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value
Liabilities carried at fair value
Multi-family collateralized debt obligations	1,052,829	0
Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	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, 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 \| Non-Agency RMBS
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 \| Multi-family loans held in securitization trusts, at fair value
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
Loans held in securitization trusts	0	0
Level 1 \| Fair Value, Measurements, Recurring \| Multi-Family CDOs
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
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	0
Total	2,022,018	1,469,815
Liabilities carried at fair value
Total	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	15,878	10,263
Level 2 \| Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	922,877	1,037,730
Level 2 \| Fair Value, Measurements, Recurring \| Agency CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	50,958	0
Level 2 \| Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	715,314	214,037
Level 2 \| Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	267,777	207,785
Level 2 \| Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities, available for sale, at fair value	49,214	0
Level 2 \| Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts, at fair value
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
Loans held in securitization trusts	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Multi-Family CDOs
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
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Level 3
Assets carried at fair value
Investments in unconsolidated entities	191,359	73,833
Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	1,429,754	737,523
Investments in unconsolidated entities	83,882	32,994
Total	20,659,268	12,503,064
Liabilities carried at fair value
Total	17,777,280	11,022,248
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 \| Non-Agency RMBS
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	52,700
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 \| Multi-family loans held in securitization trusts, at fair value
Assets carried at fair value
Loans held in securitization trusts	17,816,746	11,679,847
Level 3 \| Fair Value, Measurements, Recurring \| Residential mortgage loans held in securitization trust, at fair value
Assets carried at fair value
Loans held in securitization trusts	1,328,886	0
Level 3 \| Fair Value, Measurements, Recurring \| Multi-Family CDOs
Liabilities carried at fair value
Multi-family collateralized debt obligations	16,724,451	11,022,248
Level 3 \| Fair Value, Measurements, Recurring \| Residential collateralized debt obligations, at fair value
Liabilities carried at fair value
Multi-family collateralized debt obligations	$ 1,052,829	$ 0

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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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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