Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets		$ 5,751	$ 7,284
Carrying amounts of liabilities		5,602	6,445
Additional Voluntary Payment			66
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets		830	1,600
Carrying amounts of liabilities		$ 623	$ 1,500
Variable Interest Entity Primary Beneficiary [Member] | U S Public Finance Insurance [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated		7
Number of variable interest entities consolidated	7
Net realized gains (losses) related to the initial consolidation	$ (42)
Variable Interest Entity Primary Beneficiary [Member] | Structured Finance And International Insurance [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated		2	2
Deconsolidation, Gain (Loss), Amount			$ (16)