Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 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	$ 3,954	580
Less than 12 months Unrealized Losses	94	6
12 months or more Fair Value	820	682
12 months or more Unrealized Losses	49	18
Total Fair Value	4,774	1,262
Total Unrealized Losses	143	24
Fixed maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2,111	330
Less than 12 months Unrealized Losses	39	4
12 months or more Fair Value	336	252
12 months or more Unrealized Losses	21	9
Total Fair Value	2,447	582
Total Unrealized Losses	60	13
Fixed maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,545	88
Less than 12 months Unrealized Losses	47	1
12 months or more Fair Value	264	264
12 months or more Unrealized Losses	18	5
Total Fair Value	1,809	352
Total Unrealized Losses	65	6
Fixed maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	138	33
Less than 12 months Unrealized Losses	2	0
12 months or more Fair Value	43	36
12 months or more Unrealized Losses	2	1
Total Fair Value	181	69
Total Unrealized Losses	4	1
Fixed maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	115	96
Less than 12 months Unrealized Losses	5	1
12 months or more Fair Value	165	124
12 months or more Unrealized Losses	8	3
Total Fair Value	280	220
Total Unrealized Losses	13	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	35	23
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	12	6
12 months or more Unrealized Losses	0	0
Total Fair Value	47	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