Consolidated Securitization Vehicles and Other Variable Interest Entities - Narrative (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Non-agency MBS
Variable Interest Entity [Line Items]
Unconsolidated VIEs, aggregate investment	$ 2,000,000	$ 1,500,000
Non-agency MBS | Minimum
Variable Interest Entity [Line Items]
Unconsolidated VIEs, individual investments (Less than $1 million)	1,000	1,000
Non-agency MBS | Maximum
Variable Interest Entity [Line Items]
Unconsolidated VIEs, individual investments (Less than $1 million)	195,000	52,000
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Assets	12,356,439	13,392,951
Liabilities	7,972,942	8,652,158
Variable Interest Entities, Not Primary Beneficiary, Aggregated Disclosure
Variable Interest Entity [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount	$ 1,600,000	$ 1,200,000