Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets carried at fair value
Investment securities available for sale	$ 1,413,081	$ 818,976
Fair Value, Measurements, Recurring
Assets carried at fair value
Residential mortgage loans, at fair value	87,153	17,769
Investments in unconsolidated entities	42,823	60,332
Total	11,201,324	7,987,218
Liabilities carried at fair value
Total	9,189,459	6,625,394
Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	0
Total	0	4,506
Liabilities carried at fair value
Total	0	107
Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Residential mortgage loans, at fair value	0	0
Investments in unconsolidated entities	0	0
Total	1,366,005	920,870
Liabilities carried at fair value
Total	0	391
Level 3
Assets carried at fair value
Investments in unconsolidated entities	51,212	79,390
Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Residential mortgage loans, at fair value	87,153	17,769
Investments in unconsolidated entities	42,823	60,332
Total	9,835,319	7,061,842
Liabilities carried at fair value
Total	9,189,459	6,624,896
Agency RMBS \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	1,169,536	526,363
Agency RMBS \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
Agency RMBS \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	1,169,536	526,363
Agency RMBS \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
Non-Agency RMBS \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	102,125	163,284
Non-Agency RMBS \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
Non-Agency RMBS \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	102,125	163,284
Non-Agency RMBS \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
U.S. Treasury securities \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	2,887
U.S. Treasury securities \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	2,887
U.S. Treasury securities \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
U.S. Treasury securities \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
CMBS \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	141,420	126,442
CMBS \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	0	0
CMBS \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	93,498	82,545
CMBS \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Investment securities available for sale	47,922	43,897
Multi-family loans held in securitization trusts \| Fair Value, Measurements, Recurring
Assets carried at fair value
Multi-family loans held in securitization trusts	9,657,421	6,939,844
Multi-family loans held in securitization trusts \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Multi-family loans held in securitization trusts \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Multi-family loans held in securitization trusts	0	0
Multi-family loans held in securitization trusts \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Multi-family loans held in securitization trusts	9,657,421	6,939,844
Multi-family collateralized debt obligations, at fair value \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Multi-family collateralized debt obligations	9,189,459	6,624,896
Multi-family collateralized debt obligations, at fair value \| Level 1 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Multi-family collateralized debt obligations, at fair value \| Level 2 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Multi-family collateralized debt obligations	0	0
Multi-family collateralized debt obligations, at fair value \| Level 3 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Multi-family collateralized debt obligations	9,189,459	6,624,896
TBA Securities \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	148,139
TBA Securities \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
TBA Securities \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	148,139
TBA Securities \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
U.S. Treasury futures \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Derivative liabilities	0	107
U.S. Treasury futures \| Level 1 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Derivative liabilities	0	107
U.S. Treasury futures \| Level 2 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Derivative liabilities	0	0
U.S. Treasury futures \| Level 3 \| Fair Value, Measurements, Recurring
Liabilities carried at fair value
Derivative liabilities	0	0
Interest rate swap futures \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	444
Interest rate swap futures \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	444
Interest rate swap futures \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Interest rate swap futures \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Interest rate swaps \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	846	108
Liabilities carried at fair value
Derivative liabilities	0	391
Interest rate swaps \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Liabilities carried at fair value
Derivative liabilities	0	0
Interest rate swaps \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	846	108
Liabilities carried at fair value
Derivative liabilities	0	391
Interest rate swaps \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Liabilities carried at fair value
Derivative liabilities	0	0
Swaptions \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	431
Swaptions \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Swaptions \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	431
Swaptions \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Eurodollar futures \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	1,175
Eurodollar futures \| Level 1 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	1,175
Eurodollar futures \| Level 2 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	0	0
Eurodollar futures \| Level 3 \| Fair Value, Measurements, Recurring
Assets carried at fair value
Derivative assets	$ 0	$ 0

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Financial Instruments, Owned, Residential Mortgage Loans, at Fair Value

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This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.

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