Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Net interest income	$ 171,459	$ 141,593	$ 141,804	$ 147,439	$ 154,113	$ 147,044	$ 145,170	$ 147,881	$ 602,296	$ 594,208	$ 606,010
Total other-than-temporary impairment losses									(801)	(2,556)	(5,169)
Portion of loss recognized in other comprehensive income (loss)									(4,052)	(19,235)	(56,687)
Net other-than-temporary credit impairment losses	$ 0	$ 0	$ 0	$ (4,853)	$ (4,269)	$ (7,233)	$ (9,131)	$ (1,158)	(4,853)	(21,791)	(61,856)
Servicing fees									36,290	40,773	41,690
Variable Interest Entities, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses									(424)	(1,026)	(2,174)
Portion of loss recognized in other comprehensive income (loss)									(3,831)	(17,820)	(51,118)
Net other-than-temporary credit impairment losses									(4,255)	(18,845)	(53,292)
Servicing fees									33,920	39,313	41,493
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									780,746	904,830	914,022
Interest expense, Non-recourse liabilities of VIEs									337,387	395,255	390,858
Net interest income									$ 443,359	$ 509,575	$ 523,164