Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 522	$ 1,654
Less than 12 months Unrealized Losses	11	119
12 months or more Fair Value	406	284
12 months or more Unrealized Losses	65	37
Total Fair Value	928	1,938
Total Unrealized Losses	76	156
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	345	1,349
Less than 12 months Unrealized Losses	5	68
12 months or more Fair Value	203	284
12 months or more Unrealized Losses	10	37
Total Fair Value	548	1,633
Total Unrealized Losses	15	105
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	166	1,099
Less than 12 months Unrealized Losses	4	63
12 months or more Fair Value	200	133
12 months or more Unrealized Losses	10	33
Total Fair Value	366	1,232
Total Unrealized Losses	14	96
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	128	47
Less than 12 months Unrealized Losses	1	1
12 months or more Fair Value	0	22
12 months or more Unrealized Losses	0	0
Total Fair Value	128	69
Total Unrealized Losses	1	1
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	8	103
Less than 12 months Unrealized Losses	0	2
12 months or more Fair Value	3	2
12 months or more Unrealized Losses	0	0
Total Fair Value	11	105
Total Unrealized Losses	0	2
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	43	100
Less than 12 months Unrealized Losses	0	2
12 months or more Fair Value	0	127
12 months or more Unrealized Losses	0	4
Total Fair Value	43	227
Total Unrealized Losses	0	6
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		305
Less than 12 months Unrealized Losses		51
12 months or more Fair Value		0
12 months or more Unrealized Losses	55	0
Total Fair Value		305
Total Unrealized Losses		51
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	177	270
Less than 12 months Unrealized Losses	6	51
12 months or more Fair Value	203	0
12 months or more Unrealized Losses	55	0
Total Fair Value	380	270
Total Unrealized Losses	$ 61	51
Equity Securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		35
Less than 12 months Unrealized Losses		0
12 months or more Fair Value		0
12 months or more Unrealized Losses		0
Total Fair Value		35
Total Unrealized Losses		$ 0