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Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020	Dec. 31, 2020 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets	$ 5,375		$ 5,751
Carrying amounts of liabilities	5,403		5,602
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets	228		830
Carrying amounts of liabilities	$ 474		$ 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

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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