Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets carried at fair value
Investment securities, available for sale, at fair value	$ 1,512,252	$ 1,413,081
Level 3
Assets carried at fair value
Investments in unconsolidated entities	73,833	51,212
Fair Value, Measurements, Recurring
Assets carried at fair value
Distressed and other residential mortgage loans, at fair value	737,523	87,153
Investments in unconsolidated entities	32,994	42,823
Total	13,972,879	11,210,579
Liabilities carried at fair value
Total	11,022,248	9,189,459
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets at fair value	10,263	10,101
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	1,037,730	1,169,536
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	214,037	102,125
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	260,485	141,420
Fair Value, Measurements, Recurring \| Multi-family loans held in securitization trusts
Assets carried at fair value
Multi-family loans held in securitization trusts	11,679,847	9,657,421
Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Multi-family collateralized debt obligations	11,022,248	9,189,459
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 at fair value	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	0	0
Fair Value, Measurements, Recurring \| Level 1 \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	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, at fair value
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	1,469,815	1,375,260
Liabilities carried at fair value
Total	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Interest rate swaps
Assets carried at fair value
Derivative assets at fair value	10,263	10,101
Fair Value, Measurements, Recurring \| Level 2 \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	1,037,730	1,169,536
Fair Value, Measurements, Recurring \| Level 2 \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	214,037	102,125
Fair Value, Measurements, Recurring \| Level 2 \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	207,785	93,498
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, at fair value
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	737,523	87,153
Investments in unconsolidated entities	32,994	42,823
Total	12,503,064	9,835,319
Liabilities carried at fair value
Total	11,022,248	9,189,459
Fair Value, Measurements, Recurring \| Level 3 \| Interest rate swaps
Assets carried at fair value
Derivative assets at fair value	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Non-Agency RMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	0	0
Fair Value, Measurements, Recurring \| Level 3 \| CMBS
Assets carried at fair value
Investment securities, available for sale, at fair value	52,700	47,922
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	11,679,847	9,657,421
Fair Value, Measurements, Recurring \| Level 3 \| Multi-family collateralized debt obligations, at fair value
Liabilities carried at fair value
Multi-family collateralized debt obligations	$ 11,022,248	$ 9,189,459

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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Fair value of financial and nonfinancial obligations.

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Fair value portion of contractual obligation to pay money on demand or on fixed or determinable dates.

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