Fair Value of Financial Instruments - Valuation for Level 3 Assets (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 3,471,854	$ 20,659,268
Total gains/(losses) (realized/unrealized)
Included in earnings	4,929	(143,164)
Transfers in		500,549
Transfers out	(1,684)	(240,812)
Contributions	161	30,546
Paydowns/Distributions	(197,727)	(360,913)
Recovery of charge-off		35
Sales	(6,633)	(17,431,443)
Purchases	347,283	153,821
Balance at the end of period	3,618,183	3,167,887
Consolidated SLST \| VIE, Primary Beneficiary \| Residential loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	1,266,785	1,328,886
Total gains/(losses) (realized/unrealized)
Included in earnings	(27,057)	(89,087)
Transfers in		0
Transfers out	0	0
Contributions	0	0
Paydowns/Distributions	(28,220)	(21,500)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	1,211,508	1,218,299
Consolidated K-Series \| VIE, Primary Beneficiary \| Multi-family loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		17,816,746
Total gains/(losses) (realized/unrealized)
Included in earnings		41,795
Transfers in		0
Transfers out		(237,297)
Contributions		0
Paydowns/Distributions		(239,796)
Recovery of charge-off		35
Sales		(17,381,483)
Purchases		0
Balance at the end of period		0
Residential loans \| Residential loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	1,090,930	1,429,754
Total gains/(losses) (realized/unrealized)
Included in earnings	9,386	(94,418)
Transfers in		164,035
Transfers out	(989)	(3,166)
Contributions	0	0
Paydowns/Distributions	(125,646)	(84,719)
Recovery of charge-off		0
Sales	(4,257)	(49,960)
Purchases	347,283	153,821
Balance at the end of period	1,316,707	1,515,347
Residential loans held in securitization trusts \| Residential loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	691,451	0
Total gains/(losses) (realized/unrealized)
Included in earnings	14,123	(1,739)
Transfers in		46,572
Transfers out	(695)	(349)
Contributions	0	0
Paydowns/Distributions	(24,436)	(1,500)
Recovery of charge-off		0
Sales	(2,376)	0
Purchases	0	0
Balance at the end of period	678,067	42,984
Multi-family loans \| Multi-family loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	163,593
Total gains/(losses) (realized/unrealized)
Included in earnings	5,077
Transfers out	0
Contributions	0
Paydowns/Distributions	(16,834)
Sales	0
Purchases	0
Balance at the end of period	151,836
Preferred equity and mezzanine loan investments \| Multi-family loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		0
Total gains/(losses) (realized/unrealized)
Included in earnings		(209)
Transfers in		182,465
Transfers out		0
Contributions		8,440
Paydowns/Distributions		(11,404)
Recovery of charge-off		0
Sales		0
Purchases		0
Balance at the end of period		179,292
Equity investments \| Multi-family loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	259,095	83,882
Total gains/(losses) (realized/unrealized)
Included in earnings	3,400	494
Transfers in		107,477
Transfers out	0	0
Contributions	161	22,106
Paydowns/Distributions	(2,591)	(1,994)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	$ 260,065	$ 211,965

Fair Value, Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset, Charge-Off

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Contributions

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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-Name Accounting Standards Codification
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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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