Derivative Instruments (Credit Derivatives Sold) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	2 years 1 month 6 days	2 years 1 month 6 days
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	16 years 6 months 12 days	16 years 5 months 12 days
Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	$ 6,409	$ 6,994
Total fair value of credit derivatives	(102)	(88)
Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	2,851	3,208
Total fair value of credit derivatives	(99)	(85)
Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	3,558	3,786
Total fair value of credit derivatives	(3)	(3)
Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		0
Total fair value of credit derivatives		0
Credit Rating Aaa [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	1,774	1,947
Total fair value of credit derivatives		0
Credit Rating Aaa [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	1,774	1,947
Credit Rating Aaa [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Aaa [Member] | Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Aa [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	152	109
Total fair value of credit derivatives		0
Credit Rating Aa [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Aa [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	152	109
Credit Rating Aa [Member] | Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating A [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	2,351	3,015
Total fair value of credit derivatives	(1)	(4)
Credit Rating A [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	50	300
Credit Rating A [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	2,301	2,715
Credit Rating A [Member] | Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Bbb [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	1,084	940
Total fair value of credit derivatives	(2)	(2)
Credit Rating Bbb [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Bbb [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,084	940
Credit Rating Bbb [Member] | Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	1,048	983
Total fair value of credit derivatives	(99)	(82)
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount	1,027	961
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 21	22
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member] | Other Credit Derivatives [Member]
Derivative [Line Items]
Derivative notional amount		$ 0