Consolidated Securitization Vehicles and Other Variable Interest Entities - Narrative (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Non-agency MBS
Variable Interest Entity [Line Items]
Unconsolidated VIEs, aggregate investment	$ 1,500,000	$ 1,500,000
Non-agency MBS | Minimum
Variable Interest Entity [Line Items]
Unconsolidated VIEs, individual investments (Less than $1 million minimum)	1,000	1,000
Non-agency MBS | Maximum
Variable Interest Entity [Line Items]
Unconsolidated VIEs, individual investments (Less than $1 million minimum)	51,000	51,000
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Assets	14,693,307	10,761,954
Liabilities	10,451,235	7,300,163
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	1,400,000	$ 1,400,000
Subprime | Residential Mortgage
Variable Interest Entity [Line Items]
Mortgage loans	$ 4,100,000