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, 2021	Dec. 31, 2020
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 533	$ 406
Less than 12 months Unrealized Losses	14	8
12 months or more Fair Value	95	54
12 months or more Unrealized Losses	5	2
Total Fair Value	628	460
Total Unrealized Losses	19	10
Corporate
Investments, Unrealized Loss Position
Less than 12 months Fair Value	316	330
Less than 12 months Unrealized Losses	8	5
12 months or more Fair Value	67	46
12 months or more Unrealized Losses	3	2
Total Fair Value	383	376
Total Unrealized Losses	11	7
States, municipalities and political subdivisions
Investments, Unrealized Loss Position
Less than 12 months Fair Value	206	31
Less than 12 months Unrealized Losses	6	2
12 months or more Fair Value	14	2
12 months or more Unrealized Losses	1	0
Total Fair Value	220	33
Total Unrealized Losses	7	2
Commercial mortgage-backed
Investments, Unrealized Loss Position
Less than 12 months Fair Value	4	23
Less than 12 months Unrealized Losses	0	1
12 months or more Fair Value	14	6
12 months or more Unrealized Losses	1	0
Total Fair Value	18	29
Total Unrealized Losses	1	1
United States government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	2	12
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	2	12
Total Unrealized Losses	0	0
Foreign government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	1	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	1	10
Total Unrealized Losses	0	$ 0
Government-sponsored enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	4
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	4
Total Unrealized Losses	$ 0