Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative notional amount	$ 3,147	$ 4,043
Total fair value of credit derivatives	$ (76)	$ (66)
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	3 years	3 years 9 months
Derivative notional amount	$ 252	$ 588
Total fair value of credit derivatives	$ (74)	$ (64)
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	15 years 7 months	15 years 6 months
Derivative notional amount	$ 2,895	$ 3,035
Total fair value of credit derivatives	(2)	$ (2)
Insured Swaps - Held for Sale [Member]
Derivative [Line Items]
Weighted average remaining expected maturity		14 years 3 months
Derivative notional amount	0	$ 420
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	125	137
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	125	137
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	2,018	2,261
Total fair value of credit derivatives	(1)	(1)
Credit Rating A [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	115	115
Credit Rating A [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,903	2,146
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	847	1,152
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	847	732
Credit Rating Bbb [Member] | Insured Swaps - Held for Sale [Member]
Derivative [Line Items]
Derivative notional amount	0	420
Credit Rating Below Investment Grade [Member]
Derivative [Line Items]
Derivative notional amount	157	493
Total fair value of credit derivatives	(74)	(64)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	137	473
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 20	$ 20