Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - Fixed maturities - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 299	$ 3,031
Less than 12 months Unrealized Losses	4	70
12 months or more Fair Value	293	1,149
12 months or more Unrealized Losses	9	58
Total Fair Value	592	4,180
Total Unrealized Losses	13	128
Corporate
Investments, Unrealized Loss Position
Less than 12 months Fair Value	254	2,082
Less than 12 months Unrealized Losses	4	51
12 months or more Fair Value	198	501
12 months or more Unrealized Losses	9	36
Total Fair Value	452	2,583
Total Unrealized Losses	13	87
States, municipalities and political subdivisions
Investments, Unrealized Loss Position
Less than 12 months Fair Value	9	823
Less than 12 months Unrealized Losses	0	18
12 months or more Fair Value	13	340
12 months or more Unrealized Losses	0	13
Total Fair Value	22	1,163
Total Unrealized Losses	0	31
Commercial mortgage-backed
Investments, Unrealized Loss Position
Less than 12 months Fair Value		77
Less than 12 months Unrealized Losses		0
12 months or more Fair Value		64
12 months or more Unrealized Losses		2
Total Fair Value		141
Total Unrealized Losses		2
Government-sponsored enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	19	49
Less than 12 months Unrealized Losses	0	1
12 months or more Fair Value	78	211
12 months or more Unrealized Losses	0	6
Total Fair Value	97	260
Total Unrealized Losses	0	7
United States government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	5	0
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	4	33
12 months or more Unrealized Losses	0	1
Total Fair Value	9	33
Total Unrealized Losses	0	$ 1
Foreign government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	12
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	12
Total Unrealized Losses	$ 0