Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets		$ 2,300		$ 1,700
Carrying amounts of liabilities		$ 2,300		$ 1,700
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
Deconsolidation, Gain (Loss), Amount		$ (16)
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)