Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets carried at fair value
Investment securities available for sale	$ 1,904,018	$ 1,512,252
Interest rate swaps
Assets carried at fair value
Derivative assets		1,800
Liabilities carried at fair value
Derivative labilities	(40,400)	8,500
Variation margin	61,100	8,500
Level 3
Assets carried at fair value
Investments in unconsolidated entities	170,150	73,833
Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	1,116,128	737,523
Investments in unconsolidated entities	76,249	32,994
Total	18,980,332	13,972,879
Liabilities carried at fair value
Total	14,978,199	11,022,248
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	20,673	10,263
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	955,838	1,037,730
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	621,528	214,037
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	278,398	260,485
Fair Value, Measurements, Recurring \| ABS
Assets carried at fair value
Investment securities available for sale	48,254	0
Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	15,863,264	11,679,847
Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	14,978,199	11,022,248
Fair Value, Measurements, Recurring \| Level 1
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
Fair Value, Measurements, Recurring \| Level 1 \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 1 \| CMBS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 1 \| ABS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	0
Total	1,924,691	1,469,815
Liabilities carried at fair value
Total	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Interest rate swaps
Assets carried at fair value
Derivative assets	20,673	10,263
Fair Value, Measurements, Recurring \| Level 2 \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	955,838	1,037,730
Fair Value, Measurements, Recurring \| Level 2 \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	621,528	214,037
Fair Value, Measurements, Recurring \| Level 2 \| CMBS
Assets carried at fair value
Investment securities available for sale	278,398	207,785
Fair Value, Measurements, Recurring \| Level 2 \| ABS
Assets carried at fair value
Investment securities available for sale	48,254	0
Fair Value, Measurements, Recurring \| Level 2 \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Fair Value, Measurements, Recurring \| Level 3
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	1,116,128	737,523
Investments in unconsolidated entities	76,249	32,994
Total	17,055,641	12,503,064
Liabilities carried at fair value
Total	14,978,199	11,022,248
Fair Value, Measurements, Recurring \| Level 3 \| Interest rate swaps
Assets carried at fair value
Derivative assets	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 3 \| CMBS
Assets carried at fair value
Investment securities available for sale	0	52,700
Fair Value, Measurements, Recurring \| Level 3 \| ABS
Assets carried at fair value
Investment securities available for sale	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	15,863,264	11,679,847
Fair Value, Measurements, Recurring \| Level 3 \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	$ 14,978,199	$ 11,022,248

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Financial Instruments, Owned, Distressed And Other Residential Mortgage Loans, at Fair Value

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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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Fair value portion of investments accounted under the equity method.

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