Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets	$ 6,632		$ 7,284
Carrying amounts of liabilities	6,135		6,445
Additional Voluntary Payment			66
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets	1,200		1,600
Carrying amounts of liabilities	$ 1,200		$ 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	1