Securitized Loans Held for Investment - Roll-Forward (Detail) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - Securitized Loans Held for Investment at Fair Value - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance, beginning of period	$ 8,753,653	$ 4,768,416
Purchases	4,165,322	4,897,370
Principal paydowns	(324,851)	(1,022,414)
Sales and settlements	(1,289)	(5,007)
Net periodic accretion (amortization)	(3,109)	(41,363)
Change in fair value	120,969	146,637
Balance, end of period	$ 12,713,273	$ 8,753,653