Fair Value of Financial Instruments - Valuation for Level 3 Assets (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2021	Jun. 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	38,670	(20,886)
Transfers in		500,793
Transfers out	(2,674)	(241,599)
Transfers to securitization trust	0
Contributions	320	30,546
Paydowns/Distributions	(561,143)	(456,191)
Recovery of charge-off		35
Sales	(17,944)	(17,475,238)
Purchases	605,076	156,639
Balance at the end of period	3,534,159	3,153,367
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	(21,788)	(15,279)
Transfers in		0
Transfers out	0	0
Transfers to securitization trust	0
Contributions	0	0
Paydowns/Distributions	(68,458)	(38,757)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	1,176,539	1,274,850
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	14,494	(55,144)
Transfers in		164,279
Transfers out	(1,259)	(3,953)
Transfers to securitization trust	(160,623)
Contributions	0	0
Paydowns/Distributions	(306,209)	(155,135)
Recovery of charge-off		0
Sales	(15,568)	(93,755)
Purchases	605,076	156,639
Balance at the end of period	1,226,841	1,442,685
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	21,105	(1,730)
Transfers in		46,572
Transfers out	(1,415)	(349)
Transfers to securitization trust	160,623
Contributions	0	0
Paydowns/Distributions	(70,025)	(3,800)
Recovery of charge-off		0
Sales	(2,376)	0
Purchases	0	0
Balance at the end of period	799,363	40,693
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	10,853
Transfers out	0
Transfers to securitization trust	0
Contributions	0
Paydowns/Distributions	(47,727)
Sales	0
Purchases	0
Balance at the end of period	126,719
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		4,866
Transfers in		182,465
Transfers out		0
Contributions		8,440
Paydowns/Distributions		(14,921)
Recovery of charge-off		0
Sales		0
Purchases		0
Balance at the end of period		180,850
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		4,606
Transfers in		107,477
Contributions		22,106
Paydowns/Distributions		(3,782)
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	14,006	4,606
Transfers in		107,477
Transfers out	0	0
Transfers to securitization trust	0
Contributions	320	22,106
Paydowns/Distributions	(68,724)	(3,782)
Recovery of charge-off		0
Sales	0	0
Purchases	0	0
Balance at the end of period	$ 204,697	$ 214,289

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

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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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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 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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