Loans Held for Investment - Changes in Carrying Value of Securitized Loans Held For Investment Carried at Fair Value (Details) - Loans Held for Investment at Fair Value - Variable Interest Entities, Primary Beneficiary - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance, beginning of period	$ 13,112,129	$ 14,292,815
Purchases	2,807,700	1,860,998
Principal paydowns	(1,982,400)	(1,966,590)
Sales and settlements	(1,678,140)	(1,053,943)
Net periodic accretion (amortization)	(74,396)	(85,794)
Realized gains (losses) on sales and settlements		0
Change in fair value	348,971	64,643
Balance, end of period	$ 12,533,864	$ 13,112,129