Loans Held for Investment - Roll-Forward (Detail) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - Loans Held for Investment at Fair Value - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance, beginning of period	$ 14,292,815	$ 12,572,581	$ 12,572,581
Purchases	183,566		5,086,491
Principal paydowns	(461,594)		(1,717,745)
Sales and settlements	(171)	(1,812,760)	(1,812,760)
Net periodic accretion (amortization)	(26,284)		(77,491)
Realized gains (losses) on sales and settlements	0	1,590	1,590
Change in fair value	(681,514)	$ 240,149	240,149
Balance, end of period	$ 13,306,818		$ 14,292,815