Loans Held for Investment - Roll-Forward (Detail) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - Loans Held for Investment at Fair Value - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance, beginning of period	$ 12,572,581	$ 13,678,263
Purchases	5,086,491	1,671,330
Principal paydowns	(1,717,745)	(1,859,155)
Sales and settlements	(1,812,760)	(807,364)
Net periodic accretion (amortization)	(77,491)	(83,393)
Realized gains (losses) on sales and settlements	1,590	101
Change in fair value	240,149	(27,201)
Balance, end of period	$ 14,292,815	$ 12,572,581