Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,746	$ 2,079
Less than 12 months Unrealized Losses	60	71
12 months or more Fair Value	142	191
12 months or more Unrealized Losses	7	11
Total Fair Value	1,888	2,270
Total Unrealized Losses	67	82
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,510	1,972
Less than 12 months Unrealized Losses	51	62
12 months or more Fair Value	138	191
12 months or more Unrealized Losses	7	11
Total Fair Value	1,648	2,163
Total Unrealized Losses	58	73
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	381	733
Less than 12 months Unrealized Losses	10	15
12 months or more Fair Value	136	189
12 months or more Unrealized Losses	7	11
Total Fair Value	517	922
Total Unrealized Losses	17	26
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	883	989
Less than 12 months Unrealized Losses	37	42
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	883	989
Total Unrealized Losses	37	42
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	75	89
Less than 12 months Unrealized Losses	1	2
12 months or more Fair Value	2	2
12 months or more Unrealized Losses	0	0
Total Fair Value	77	91
Total Unrealized Losses	1	2
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	165	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	165	155
Total Unrealized Losses	3	3
Fixed Maturities | United States government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	6	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	6	6
Total Unrealized Losses	0	0
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	236	107
Less than 12 months Unrealized Losses	9	9
12 months or more Fair Value	4	0
12 months or more Unrealized Losses	0	0
Total Fair Value	240	107
Total Unrealized Losses	9	9
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	236	103
Less than 12 months Unrealized Losses	9	9
12 months or more Fair Value	4	0
12 months or more Unrealized Losses	0	0
Total Fair Value	240	103
Total Unrealized Losses	$ 9	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