Derivative Instruments - Summary of Gross Principal Notional Outstanding for CDS Contracts (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Member] | Interest Rate Swap [Member]
Ambac Rating
Derivative, Notional Amount	$ 1,233	$ 1,194
Variable Interest Entity [Member] | Interest Rate Swap One [Member]
Ambac Rating
Derivative, Notional Amount	1,151	1,176
Variable Interest Entity [Member] | Currency Swaps [Member]
Ambac Rating
Derivative, Notional Amount	308	329
Variable Interest Entity [Member] | Other Credit Derivatives [Member]
Ambac Rating
Derivative, Notional Amount	0	9
Other Credit Derivatives [Member]
Ambac Rating
Derivative, Notional Amount	$ 257	$ 280