Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets			$ 5,252	$ 5,751
Carrying amounts of liabilities			5,275	5,602
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets			218	830
Carrying amounts of liabilities			$ 450	$ 623
Variable Interest Entity Primary Beneficiary [Member] | US Public Finance [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated		7
Number of variable interest entities consolidated		0
Variable Interest Entity Primary Beneficiary [Member] | Structured Finance And International Insurance [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated	0	1	2
Variable Interest Entity Primary Beneficiary [Member] | Structured Finance And International Insurance [Member] | Recorded Due To Credit Losses In AOCI [Member]
Variable Interest Entity [Line Items]
Net realized gains (losses) related to deconsolidation			$ (5)