Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative notional amount	$ 1,718	$ 1,827
Total fair value of credit derivatives	$ (10)	$ (9)
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	9 months 18 days	1 year
Derivative notional amount	$ 24	$ 32
Total fair value of credit derivatives	$ (8)	$ (7)
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	15 years	14 years 7 months 6 days
Derivative notional amount	$ 1,694	$ 1,795
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	64	66
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	64	66
Credit Rating A [Member]
Derivative [Line Items]
Derivative notional amount	1,256	1,284
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	1,256	1,284
Credit Rating Bbb [Member]
Derivative [Line Items]
Derivative notional amount	374	445
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	374	445
Credit Rating Below Investment Grade [Member]
Derivative [Line Items]
Derivative notional amount	24	32
Total fair value of credit derivatives	(8)	(7)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	24	32
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 0	$ 0