Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets carried at fair value
Investment securities available for sale	$ 1,290,015	$ 1,413,081
Level 3
Assets carried at fair value
Investments in unconsolidated entities	53,737	51,212
Fair Value, Measurements, Recurring
Assets carried at fair value
Residential mortgage loans, at fair value	169,197	87,153
Investments in unconsolidated entities	45,152	42,823
Total	10,860,267	11,210,579
Liabilities carried at fair value
Total	8,838,841	9,189,459
Fair Value, Measurements, Recurring \| Interest rate swaps
Assets carried at fair value
Derivative assets	10,543	10,101
Fair Value, Measurements, Recurring \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	1,101,344	1,169,536
Fair Value, Measurements, Recurring \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	54,057	102,125
Fair Value, Measurements, Recurring \| CMBS
Assets carried at fair value
Investment securities available for sale	134,614	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	9,345,360	9,657,421
Fair Value, Measurements, Recurring \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	8,838,841	9,189,459
Fair Value, Measurements, Recurring \| Level 1
Assets carried at fair value
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
Residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	0
Total	1,250,424	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	10,543	10,101
Fair Value, Measurements, Recurring \| Level 2 \| Agency RMBS
Assets carried at fair value
Investment securities available for sale	1,101,344	1,169,536
Fair Value, Measurements, Recurring \| Level 2 \| Non-Agency RMBS
Assets carried at fair value
Investment securities available for sale	54,057	102,125
Fair Value, Measurements, Recurring \| Level 2 \| CMBS
Assets carried at fair value
Investment securities available for sale	84,480	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
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Fair Value, Measurements, Recurring \| Level 3
Assets carried at fair value
Residential mortgage loans, at fair value	169,197	87,153
Investments in unconsolidated entities	45,152	42,823
Total	9,609,843	9,835,319
Liabilities carried at fair value
Total	8,838,841	9,189,459
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	50,134	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	9,345,360	9,657,421
Fair Value, Measurements, Recurring \| Level 3 \| Multi-family collateralized debt obligations
Liabilities carried at fair value
Multi-family collateralized debt obligations	$ 8,838,841	$ 9,189,459

Financial Instruments, Owned, 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, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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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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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as held-to-maturity.

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