Variable Interest Entities (Narrative) (Detail) $ in Millions		3 Months Ended	12 Months Ended
	Jan. 27, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets			$ 7,284	$ 8,107
Carrying amounts of liabilities			6,445	6,975
Additional Voluntary Payment			66
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Carrying amounts of assets			1,600	1,700
Carrying amounts of liabilities			$ 1,500	$ 1,700
Variable Interest Entity Primary Beneficiary [Member] | Subsequent Event [Member]
Variable Interest Entity [Line Items]
Reduction In Variable Interest Entity Asset And Liabilities	$ 315
Variable Interest Entity Primary Beneficiary [Member] | U S Public Finance Insurance [Member]
Variable Interest Entity [Line Items]
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			2	8
Deconsolidation, Gain (Loss), Amount			$ (16)	$ (78)
Variable Interest Entity Primary Beneficiary [Member] | Structured Finance And International Insurance [Member] | Subsequent Event [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated	1
Variable Interest Entity Primary Beneficiary [Member] | Structured Finance And International Insurance [Member] | Zohar Funds Bankruptcy Cases Debtors [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated				2
Deconsolidation, Gain (Loss), Amount				$ (93)