Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,559	$ 2,079
Less than 12 months Unrealized Losses	45	71
12 months or more Fair Value	141	191
12 months or more Unrealized Losses	6	11
Total Fair Value	1,700	2,270
Total Unrealized Losses	51	82
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,205	1,972
Less than 12 months Unrealized Losses	29	62
12 months or more Fair Value	141	191
12 months or more Unrealized Losses	6	11
Total Fair Value	1,346	2,163
Total Unrealized Losses	35	73
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	290	733
Less than 12 months Unrealized Losses	8	15
12 months or more Fair Value	136	189
12 months or more Unrealized Losses	6	11
Total Fair Value	426	922
Total Unrealized Losses	14	26
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	665	989
Less than 12 months Unrealized Losses	17	42
12 months or more Fair Value	2	0
12 months or more Unrealized Losses	0	0
Total Fair Value	667	989
Total Unrealized Losses	17	42
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	53	89
Less than 12 months Unrealized Losses	1	2
12 months or more Fair Value	3	2
12 months or more Unrealized Losses	0	0
Total Fair Value	56	91
Total Unrealized Losses	1	2
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	190	155
Less than 12 months Unrealized Losses	3	3
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	190	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	354	107
Less than 12 months Unrealized Losses	16	9
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	354	107
Total Unrealized Losses	16	9
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	354	103
Less than 12 months Unrealized Losses	16	9
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	354	103
Total Unrealized Losses	$ 16	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