Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets carried at fair value
Investment securities available for sale	$ 1,583,965	$ 1,512,252
Interest rate swaps
Assets carried at fair value
Derivative assets		1,800
Liabilities carried at fair value
Derivative labilities	12,800	8,500
Variation margin	27,700	8,500
Level 3
Assets carried at fair value
Investments in unconsolidated entities	92,961	73,833
Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	875,566	737,523
Investments in unconsolidated entities	36,575	32,994
Total	16,839,315	13,972,879
Liabilities carried at fair value
Total	13,547,195	11,022,248
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	14,873	10,263
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	1,023,938	1,037,730
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	314,086	214,037
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	245,941	260,485
Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	14,328,336	11,679,847
Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	13,547,195	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 \| 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	1,598,838	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	14,873	10,263
Fair Value, Measurements, Recurring \| Level 2 \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	1,023,938	1,037,730
Fair Value, Measurements, Recurring \| Level 2 \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	314,086	214,037
Fair Value, Measurements, Recurring \| Level 2 \| CMBS
Assets carried at fair value
Investment securities available for sale	245,941	207,785
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	875,566	737,523
Investments in unconsolidated entities	36,575	32,994
Total	15,240,477	12,503,064
Liabilities carried at fair value
Total	13,547,195	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 \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	14,328,336	11,679,847
Fair Value, Measurements, Recurring \| Level 3 \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	$ 13,547,195	$ 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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