Fair Value of Financial Instruments - Valuation for Level 3 Assets (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 12,503,064	$ 9,835,319
Total gains/(losses) (realized/unrealized)
Included in earnings	829,948	(280,300)
Included in other comprehensive income (loss)	(13,665)	901
Transfers in	0	0
Transfers out	(437)	0
Contributions	50,000	0
Paydowns/Distributions	(488,838)	(133,101)
Charge-off	(3,510)
Sales	(76,583)	(7,105)
Purchases	4,255,662	923,842
Balance at the end of period	17,055,641	10,339,556
Assets of consolidated VIEs	15,976,914		$ 11,984,374
Consolidated K-Series
Total gains/(losses) (realized/unrealized)
Assets of consolidated VIEs	3,800,000		$ 800,000
Multi-family loans held in securitization trusts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	11,679,847	9,657,421
Total gains/(losses) (realized/unrealized)
Included in earnings	760,132	(289,797)
Included in other comprehensive income (loss)	0	0
Transfers in	0	0
Transfers out	0	0
Contributions	0	0
Paydowns/Distributions	(368,811)	(101,953)
Charge-off	(3,510)	0
Sales	0	0
Purchases	3,795,606	805,163
Balance at the end of period	15,863,264	10,070,834
Distressed and other residential mortgage loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	737,523	87,153
Total gains/(losses) (realized/unrealized)
Included in earnings	44,913	(1,361)
Included in other comprehensive income (loss)	0	0
Transfers in	0	0
Transfers out	(437)	0
Contributions	0	0
Paydowns/Distributions	(106,113)	(15,456)
Charge-off	0
Sales	(19,814)	(7,105)
Purchases	460,056	118,679
Balance at the end of period	1,116,128	181,910
Investments in unconsolidated entities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	32,994	42,823
Total gains/(losses) (realized/unrealized)
Included in earnings	7,169	7,930
Included in other comprehensive income (loss)	0	0
Transfers in	0	0
Transfers out	0	0
Contributions	50,000	0
Paydowns/Distributions	(13,914)	(15,692)
Charge-off	0
Sales	0	0
Purchases	0	0
Balance at the end of period	76,249	35,061
CMBS held in securitization trusts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	52,700	47,922
Total gains/(losses) (realized/unrealized)
Included in earnings	17,734	2,928
Included in other comprehensive income (loss)	(13,665)	901
Transfers in	0	0
Transfers out	0	0
Contributions	0	0
Paydowns/Distributions	0	0
Charge-off	0
Sales	(56,769)	0
Purchases	0	0
Balance at the end of period	$ 0	$ 51,751

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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Amount of gain (loss) recognized in other comprehensive income (OCI) 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 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 settlement 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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The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

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