Variable Interest Entities (Narrative) (Detail) $ in Millions	1 Months Ended	3 Months Ended			12 Months Ended
	Oct. 31, 2020 USD ($) issue	Mar. 31, 2020	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets					$ 7,284	$ 6,551
Carrying amounts of liabilities					6,445	6,308
Additional Voluntary Payment					66
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets					1,600	1,300
Carrying amounts of liabilities					$ 1,500	$ 1,200
Variable Interest Entity Primary Beneficiary [Member] | Subsequent Event [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated | issue	1
Reduction in Variable Interest Entities assets and liabilities	$ 575
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		1	2
Deconsolidation, Gain (Loss), Amount			$ (16)