Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,217	$ 2,079
Less than 12 months Unrealized Losses	53	71
12 months or more Fair Value	361	191
12 months or more Unrealized Losses	12	11
Total Fair Value	1,578	2,270
Total Unrealized Losses	65	82
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	941	1,972
Less than 12 months Unrealized Losses	15	62
12 months or more Fair Value	361	191
12 months or more Unrealized Losses	12	11
Total Fair Value	1,302	2,163
Total Unrealized Losses	27	73
Fixed maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	231	733
Less than 12 months Unrealized Losses	4	15
12 months or more Fair Value	193	189
12 months or more Unrealized Losses	7	11
Total Fair Value	424	922
Total Unrealized Losses	11	26
Fixed maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	508	989
Less than 12 months Unrealized Losses	8	42
12 months or more Fair Value	122	0
12 months or more Unrealized Losses	5	0
Total Fair Value	630	989
Total Unrealized Losses	13	42
Fixed maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	43	89
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	46	91
Total Unrealized Losses	0	2
Fixed maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	152	155
Less than 12 months Unrealized Losses	3	3
12 months or more Fair Value	43	0
12 months or more Unrealized Losses	0	0
Total Fair Value	195	155
Total Unrealized Losses	3	3
Fixed maturities | United States government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	7	6
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	7	6
Total Unrealized Losses	0	0
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	276	107
Less than 12 months Unrealized Losses	38	9
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	276	107
Total Unrealized Losses	38	9
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	276	103
Less than 12 months Unrealized Losses	38	9
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	276	103
Total Unrealized Losses	$ 38	9
Equity securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		4
Less than 12 months Unrealized Losses		0
12 months or more Fair Value		0
12 months or more Unrealized Losses		0
Total Fair Value		4
Total Unrealized Losses		$ 0