Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative notional amount	$ 2,116	$ 2,503
Total fair value of credit derivatives	$ (21)	$ (35)
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	9 months 18 days	1 year
Derivative notional amount	$ 53	$ 70
Total fair value of credit derivatives	$ (19)	$ (33)
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	15 years 6 months	15 years 8 months 12 days
Derivative notional amount	$ 2,063	$ 2,433
Total fair value of credit derivatives	(2)	(2)
Insured Swaps - Held for Sale [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Aaa [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Total fair value of credit derivatives	0	0
Credit Rating Aaa [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Aaa [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Aaa [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount	0
Credit Rating Aa [Member]
Derivative [Line Items]
Derivative notional amount	72	74
Total fair value of credit derivatives	0	0
Credit Rating Aa [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Aa [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	72	74
Credit Rating Aa [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount	0
Credit Rating A [Member]
Derivative [Line Items]
Derivative notional amount	1,436	1,463
Total fair value of credit derivatives	(1)	(1)
Credit Rating A [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating A [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,436	1,463
Credit Rating A [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount	0
Credit Rating Bbb [Member]
Derivative [Line Items]
Derivative notional amount	555	896
Total fair value of credit derivatives	(1)	(1)
Credit Rating Bbb [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Bbb [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	555	896
Credit Rating Bbb [Member] | Insured Swaps - Held for Sale [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Below Investment Grade [Member]
Derivative [Line Items]
Derivative notional amount	53	70
Total fair value of credit derivatives	(19)	(33)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	53	70
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 0	$ 0