Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		$ 809
Less than 12 months Unrealized Losses		20
12 months or more Fair Value		682
12 months or more Unrealized Losses		18
Total Fair Value		1,491
Total Unrealized Losses		38
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 2,300	580
Less than 12 months Unrealized Losses	45	6
12 months or more Fair Value	678	682
12 months or more Unrealized Losses	34	18
Total Fair Value	2,978	1,262
Total Unrealized Losses	79	24
Fixed maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,002	330
Less than 12 months Unrealized Losses	20	4
12 months or more Fair Value	253	252
12 months or more Unrealized Losses	15	9
Total Fair Value	1,255	582
Total Unrealized Losses	35	13
Fixed maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,052	88
Less than 12 months Unrealized Losses	21	1
12 months or more Fair Value	256	264
12 months or more Unrealized Losses	14	5
Total Fair Value	1,308	352
Total Unrealized Losses	35	6
Fixed maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	81	33
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	35	36
12 months or more Unrealized Losses	1	1
Total Fair Value	116	69
Total Unrealized Losses	2	1
Fixed maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	131	96
Less than 12 months Unrealized Losses	2	1
12 months or more Fair Value	123	124
12 months or more Unrealized Losses	4	3
Total Fair Value	254	220
Total Unrealized Losses	6	4
Fixed maturities | Foreign government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	10	10
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	10	10
Total Unrealized Losses	0	0
Fixed maturities | United States government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	24	23
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	11	6
12 months or more Unrealized Losses	0	0
Total Fair Value	35	29
Total Unrealized Losses	$ 1	0
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		229
Less than 12 months Unrealized Losses		14
12 months or more Fair Value		0
12 months or more Unrealized Losses		0
Total Fair Value		229
Total Unrealized Losses		14
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		229
Less than 12 months Unrealized Losses		14
12 months or more Fair Value		0
12 months or more Unrealized Losses		0
Total Fair Value		229
Total Unrealized Losses		$ 14