Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 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	$ 3,162	580
Less than 12 months Unrealized Losses	66	6
12 months or more Fair Value	749	682
12 months or more Unrealized Losses	40	18
Total Fair Value	3,911	1,262
Total Unrealized Losses	106	24
Fixed maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,836	330
Less than 12 months Unrealized Losses	39	4
12 months or more Fair Value	283	252
12 months or more Unrealized Losses	18	9
Total Fair Value	2,119	582
Total Unrealized Losses	57	13
Fixed maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,070	88
Less than 12 months Unrealized Losses	22	1
12 months or more Fair Value	262	264
12 months or more Unrealized Losses	14	5
Total Fair Value	1,332	352
Total Unrealized Losses	36	6
Fixed maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	113	33
Less than 12 months Unrealized Losses	2	0
12 months or more Fair Value	35	36
12 months or more Unrealized Losses	2	1
Total Fair Value	148	69
Total Unrealized Losses	4	1
Fixed maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	110	96
Less than 12 months Unrealized Losses	2	1
12 months or more Fair Value	158	124
12 months or more Unrealized Losses	6	3
Total Fair Value	268	220
Total Unrealized Losses	8	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	23	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	34	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