Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 2,258	$ 1,384
Less than 12 Months, Unrealized Loss	23	18
12 months or more, Fair Value	7,799	7,912
12 months or longer, Unrealized Loss	691	669
Total Fair Value	10,057	9,296
Total Unrealized Losses	714	687
Corporate
Investments, Unrealized Loss Position
Less than 12 months Fair Value	642	379
Less than 12 Months, Unrealized Loss	13	13
12 months or more, Fair Value	5,488	5,560
12 months or longer, Unrealized Loss	450	441
Total Fair Value	6,130	5,939
Total Unrealized Losses	463	454
States, municipalities and political subdivisions
Investments, Unrealized Loss Position
Less than 12 months Fair Value	1,099	313
Less than 12 Months, Unrealized Loss	9	2
12 months or more, Fair Value	1,900	1,932
12 months or longer, Unrealized Loss	223	206
Total Fair Value	2,999	2,245
Total Unrealized Losses	232	208
Government-sponsored enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	396	652
Less than 12 Months, Unrealized Loss	1	3
12 months or more, Fair Value	168	113
12 months or longer, Unrealized Loss	3	3
Total Fair Value	564	765
Total Unrealized Losses	4	6
Asset-backed
Investments, Unrealized Loss Position
Less than 12 months Fair Value	30	5
Less than 12 Months, Unrealized Loss	0	0
12 months or more, Fair Value	133	172
12 months or longer, Unrealized Loss	11	16
Total Fair Value	163	177
Total Unrealized Losses	11	16
United States government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	74	32
Less than 12 Months, Unrealized Loss	0	0
12 months or more, Fair Value	105	129
12 months or longer, Unrealized Loss	4	3
Total Fair Value	179	161
Total Unrealized Losses	4	3
Foreign government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	17	3
Less than 12 Months, Unrealized Loss	0	0
12 months or more, Fair Value	5	6
12 months or longer, Unrealized Loss	0	0
Total Fair Value	22	9
Total Unrealized Losses	$ 0	$ 0