Consolidated Securitization Vehicles and Other Variable Interest Entities - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Variable Interest Entity [Line Items]
Net interest income	$ 145,170	$ 150,689	$ 293,049	$ 291,802
Total other-than-temporary impairment losses	(805)	(749)	(1,099)	(3,462)
Portion of loss recognized in other comprehensive income (loss)	(8,326)	(12,760)	(9,190)	(28,748)
Net other-than-temporary credit impairment losses	(9,131)	(13,509)	(10,289)	(32,210)
Servicing fees	9,943	10,890	21,277	20,478
Variable Interest Entities, Primary Beneficiary
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses	(351)	(696)	(351)	(1,109)
Portion of loss recognized in other comprehensive income (loss)	(8,561)	(12,146)	(8,561)	(25,613)
Net other-than-temporary credit impairment losses	(8,912)	(12,842)	(8,912)	(26,722)
Servicing fees	9,722	10,890	20,133	20,478
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	229,746	234,437	464,772	427,426
Interest expense, Non-recourse liabilities of VIEs	99,507	105,723	199,121	188,407
Net interest income	$ 130,239	$ 128,714	$ 265,651	$ 239,019