Derivative Instruments (Credit Derivatives Sold) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Credit Default Swap [Member]
Derivative [Line Items]
Weighted average remaining expected maturity		1 year
Insured Swaps [Member]
Derivative [Line Items]
Weighted average remaining expected maturity	13 years 10 months 24 days	14 years
Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	$ 1,743	$ 1,957
Total fair value of credit derivatives	(2)	(9)
Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		32
Total fair value of credit derivatives		(7)
Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,743	1,925	[1]
Total fair value of credit derivatives	(2)	(2)
Credit Rating Aaa [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Total fair value of credit derivatives	0	0
Credit Rating Aaa [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating Aaa [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	0	0
Credit Rating Aa [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	58	121
Total fair value of credit derivatives	0	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	58	121	[2]
Credit Rating A [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	1,327	1,371
Total fair value of credit derivatives	(1)	(1)
Credit Rating A [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		0
Credit Rating A [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,327	1,371	[3]
Credit Rating Bbb [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	358	433
Total fair value of credit derivatives	(1)	(1)
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	358	433	[4]
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member]
Derivative [Line Items]
Derivative notional amount	0	32
Total fair value of credit derivatives		(7)
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member] | Credit Default Swap [Member]
Derivative [Line Items]
Derivative notional amount		32
Credit Rating Below Investment Grade [Member] | Insurance Operations [Member] | Insured Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 0	$ 0

[1] The Company revised its previously reported amount of $1,795 million to $1,925 million.

[2] The Company revised its previously reported amount of $66 million to $121 million.

[3] The Company revised its previously reported amount of $1,284 million to $1,371 million.

[4] The Company revised its previously reported amount of $445 million to $433 million.