Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - Fixed maturities - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 362	$ 3,031
Less than 12 months Unrealized Losses	5	70
12 months or more Fair Value	1,194	1,149
12 months or more Unrealized Losses	27	58
Total Fair Value	1,556	4,180
Total Unrealized Losses	32	128
Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	343	2,082
Less than 12 months Unrealized Losses	5	51
12 months or more Fair Value	685	501
12 months or more Unrealized Losses	20	36
Total Fair Value	1,028	2,583
Total Unrealized Losses	25	87
States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	10	823
Less than 12 months Unrealized Losses	0	18
12 months or more Fair Value	237	340
12 months or more Unrealized Losses	3	13
Total Fair Value	247	1,163
Total Unrealized Losses	3	31
Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	7	49
Less than 12 months Unrealized Losses	0	1
12 months or more Fair Value	204	211
12 months or more Unrealized Losses	3	6
Total Fair Value	211	260
Total Unrealized Losses	3	7
Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2	77
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	45	64
12 months or more Unrealized Losses	1	2
Total Fair Value	47	141
Total Unrealized Losses	1	2
United States government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	23	33
12 months or more Unrealized Losses	0	1
Total Fair Value	23	33
Total Unrealized Losses	$ 0	$ 1