Fair Value of Financial Instruments - Valuation for Level 3 Assets (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Sep. 30, 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	83,305	72,991
Transfers in		500,793
Transfers out	(3,787)	(243,855)
Transfers to securitization trust	0	0
Funding/Contributions	53,200	49,663
Paydowns/Distributions	(864,268)	(588,320)
Recovery of charge-off		35
Sales	(67,452)	(17,475,238)
Purchases	975,781	249,312
Balance at the end of period	3,648,633	3,224,649
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	(16,522)	24,096
Transfers in		0
Transfers out	0	0
Transfers to securitization trust	0	0
Funding/Contributions	0	0
Paydowns/Distributions	(113,258)	(62,977)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	1,137,005	1,290,005
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)
Transfers to securitization trust		0
Funding/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	28,094	(26,069)
Transfers in		164,279
Transfers out	(2,080)	(5,061)
Transfers to securitization trust	(236,751)	(317,089)
Funding/Contributions	0	0
Paydowns/Distributions	(464,097)	(224,073)
Recovery of charge-off		0
Sales	(65,076)	(93,755)
Purchases	974,199	249,312
Balance at the end of period	1,325,219	1,177,298
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	34,901	8,836
Transfers in		46,572
Transfers out	(1,707)	(1,497)
Transfers to securitization trust	(236,751)	317,089
Funding/Contributions	0	0
Paydowns/Distributions	(149,019)	(15,514)
Recovery of charge-off		0
Sales	(2,376)	0
Purchases	1,582	0
Balance at the end of period	811,583	355,486
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	14,811
Transfers out	0
Transfers to securitization trust	0
Funding/Contributions	7,970
Paydowns/Distributions	(66,562)
Sales	0
Purchases	0
Balance at the end of period	119,812
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		9,760
Transfers in		182,465
Transfers out		0
Transfers to securitization trust		0
Funding/Contributions		13,712
Paydowns/Distributions		(22,783)
Recovery of charge-off		0
Sales		0
Purchases		0
Balance at the end of period		183,154
Equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		83,882
Total gains/(losses) (realized/unrealized)
Included in earnings		14,573
Transfers in		107,477
Funding/Contributions		35,951
Paydowns/Distributions		(23,177)
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	22,021	14,573
Transfers in		107,477
Transfers out	0	0
Transfers to securitization trust	0	0
Funding/Contributions	45,230	35,951
Paydowns/Distributions	(71,332)	(23,177)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	$ 255,014	$ 218,706

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, Funding/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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-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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- Definition

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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