Consolidated Securitization Vehicles and Other Variable Interest Entities - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Non-agency MBS
Variable Interest Entity [Line Items]
Unconsolidated VIEs, aggregate investment	$ 1,500,000	$ 1,300,000
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)	52,000	52,000
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Assets	13,392,951	14,987,464
Liabilities	8,652,158	9,631,820
Variable Interest Entities, Not Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	1,200,000	1,200,000
Subprime | Residential Mortgage
Variable Interest Entity [Line Items]
Financing receivable, significant purchases	$ 2,200,000	$ 1,300,000