Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets	$ 4,816				$ 5,751
Carrying amounts of liabilities	4,973				5,602
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets	168				830
Carrying amounts of liabilities	$ 292				$ 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	1	2	0	1
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	$ (10)	$ (5)