Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Variable Interest Entity [Line Items]
Net interest income	$ 141,113	$ 138,213
Total other-than-temporary impairment losses	(2,713)	(4,423)
Portion of loss recognized in other comprehensive income	(15,988)	(6,255)
Net other-than-temporary credit impairment losses	(18,701)	(10,678)
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses	(413)	(233)
Portion of loss recognized in other comprehensive income	(13,467)	(7,864)
Net other-than-temporary credit impairment losses	(13,880)	(8,097)
Non Agency Residential Mortgage Backed Securities And Securitized Loans | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Interest income, Assets of consolidated VIEs	192,989	131,980
Interest expense, Non-recourse liabilities of VIEs	82,684	39,250
Net interest income	$ 110,305	$ 92,730