Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative notional amount	$ 2,638	$ 2,928
Total fair value of credit derivatives	$ (29)	$ (65)
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	3 months 18 days	1 year
Derivative notional amount	$ 77	$ 127
Total fair value of credit derivatives	$ (27)	$ (63)
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	15 years 6 months	15 years 6 months
Derivative notional amount	$ 2,561	$ 2,801
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	107	117
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	107	117
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	1,539	1,818
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	1,539	1,818
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	915	846
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	915	846
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	77	147
Total fair value of credit derivatives	(27)	(63)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	77	127
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 0	$ 20