Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Variable Interest Entity [Line Items]
Net interest income	$ 156,455	$ 156,042	$ 448,257	$ 432,126
Total other-than-temporary impairment losses	(784)	(993)	(4,245)	(8,555)
Portion of loss recognized in other comprehensive income (loss)	(10,684)	(10,581)	(39,431)	(34,652)
Net other-than-temporary credit impairment losses	(11,468)	(11,574)	(43,676)	(43,207)
Servicing fees	10,715	9,788	31,193	23,139
Variable Interest Entities, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses	(668)	(21)	(1,777)	(866)
Portion of loss recognized in other comprehensive income (loss)	(10,108)	(4,599)	(35,720)	(31,925)
Net other-than-temporary credit impairment losses	(10,776)	(4,620)	(37,497)	(32,791)
Servicing fees	10,492	9,788	30,970	23,139
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	241,195	195,488	668,621	488,353
Interest expense, Non-recourse liabilities of VIEs	101,856	70,715	290,264	168,738
Net interest income	$ 139,339	$ 124,773	$ 378,357	$ 319,615