Variable Interest Entities (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Carrying amounts of assets	$ 2,400	$ 1,700
Carrying amounts of liabilities	$ 2,600	$ 1,700
Variable Interest Entity Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of variable interest entities deconsolidated	0
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)