Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative notional amount	$ 1,856	$ 2,503
Total fair value of credit derivatives	$ (12)	$ (35)
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	3 months 18 days	1 year
Derivative notional amount	$ 34	$ 70
Total fair value of credit derivatives	$ (10)	$ (33)
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	14 years 9 months 18 days	15 years 8 months 12 days
Derivative notional amount	$ 1,822	$ 2,433
Total fair value of credit derivatives	(2)	(2)
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 Aa [Member]
Derivative [Line Items]
Derivative notional amount	66	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	66	74
Credit Rating A [Member]
Derivative [Line Items]
Derivative notional amount	1,295	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	1,295	1,463
Credit Rating Bbb [Member]
Derivative [Line Items]
Derivative notional amount	461	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	461	896
Credit Rating Below Investment Grade [Member]
Derivative [Line Items]
Derivative notional amount	34	70
Total fair value of credit derivatives	(10)	(33)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	34	70
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 0	$ 0