Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Net interest income	$ 154,086	$ 156,042	$ 137,869	$ 138,213	$ 136,957	$ 143,966	$ 149,760	$ 182,689	$ 586,211	$ 613,372	$ 540,010
Total other-than-temporary impairment losses	(1,034)	(993)	(3,139)	(4,423)	(2,311)	(3,129)	(2,208)	(1,052)	(9,589)	(8,700)	(8,713)
Portion of loss recognized in other comprehensive income	(13,746)	(10,581)	(17,816)	(6,255)	(12,385)	(14,703)	(24,893)	(6,763)	(48,398)	(58,744)	(55,279)
Net other-than-temporary credit impairment losses	$ (14,780)	$ (11,574)	$ (20,955)	$ (10,678)	$ (14,696)	$ (17,832)	$ (27,101)	$ (7,815)	(57,987)	(67,444)	(63,992)
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses									(1,274)	(2,066)	(3,374)
Portion of loss recognized in other comprehensive income									(37,475)	(55,552)	(55,392)
Net other-than-temporary credit impairment losses									(38,749)	(57,618)	(58,766)
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									678,623	575,715	428,992
Interest expense, Non-recourse liabilities of VIEs									(249,708)	(191,922)	(119,103)
Net interest income									$ 428,915	$ 383,793	$ 309,889