Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Variable Interest Entity [Line Items]
Net interest income	$ 150,689	$ 137,869	$ 291,802	$ 276,085
Total other-than-temporary impairment losses	(749)	(3,139)	(3,462)	(7,562)
Portion of loss recognized in other comprehensive income	(12,760)	(17,816)	(28,748)	(24,071)
Net other-than-temporary credit impairment losses	(13,509)	(20,955)	(32,210)	(31,633)
Servicing fees	10,890	7,773	20,478	13,351
Variable Interest Entities, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses	(696)	(612)	(1,109)	(845)
Portion of loss recognized in other comprehensive income	(12,146)	(19,462)	(25,613)	(27,326)
Net other-than-temporary credit impairment losses	(12,842)	(20,074)	(26,722)	(28,171)
Non Agency Residential Mortgage Backed Securities And Securitized Loans | Variable Interest Entities, Primary Beneficiary
Variable Interest Entity [Line Items]
Interest income, Assets of consolidated VIEs	234,437	160,885	427,426	292,865
Interest expense, Non-recourse liabilities of VIEs	105,723	58,772	188,407	98,022
Net interest income	$ 128,714	$ 102,113	$ 239,019	$ 194,843