Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2019 USD ($)	Sep. 30, 2018	Jun. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets					$ 1,600	$ 1,700
Carrying amounts of liabilities					$ 1,600	$ 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		3
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)