Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	$ 4,653	$ 3,483
Gross Unrealized Losses, Less than 12 Months	87	74
Estimated Fair Value, 12 Months or Longer	22,359	22,248
Gross Unrealized Losses, 12 Months or Longer	3,009	2,921
Estimated Fair Value, Total	27,012	25,731
Gross Unrealized Losses, Total	3,096	2,995
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	2,578	1,943
Gross Unrealized Losses, Less than 12 Months	48	37
Estimated Fair Value, 12 Months or Longer	13,459	13,406
Gross Unrealized Losses, 12 Months or Longer	1,326	1,308
Estimated Fair Value, Total	16,037	15,349
Gross Unrealized Losses, Total	1,374	1,345
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	686	598
Gross Unrealized Losses, Less than 12 Months	17	18
Estimated Fair Value, 12 Months or Longer	3,266	3,104
Gross Unrealized Losses, 12 Months or Longer	739	685
Estimated Fair Value, Total	3,952	3,702
Gross Unrealized Losses, Total	756	703
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	434	233
Gross Unrealized Losses, Less than 12 Months	8	4
Estimated Fair Value, 12 Months or Longer	2,159	2,212
Gross Unrealized Losses, 12 Months or Longer	475	421
Estimated Fair Value, Total	2,593	2,445
Gross Unrealized Losses, Total	483	425
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	131	200
Gross Unrealized Losses, Less than 12 Months	2	5
Estimated Fair Value, 12 Months or Longer	1,162	1,184
Gross Unrealized Losses, 12 Months or Longer	191	225
Estimated Fair Value, Total	1,293	1,384
Gross Unrealized Losses, Total	193	230
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	588	392
Gross Unrealized Losses, Less than 12 Months	9	8
Estimated Fair Value, 12 Months or Longer	1,833	1,869
Gross Unrealized Losses, 12 Months or Longer	242	248
Estimated Fair Value, Total	2,421	2,261
Gross Unrealized Losses, Total	251	256
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,153	825
Gross Unrealized Losses, Less than 12 Months	19	17
Estimated Fair Value, 12 Months or Longer	5,154	5,265
Gross Unrealized Losses, 12 Months or Longer	908	894
Estimated Fair Value, Total	6,307	6,090
Gross Unrealized Losses, Total	927	911
U.S. Treasury and obligations of government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	100	65
Gross Unrealized Losses, Less than 12 Months	1	1
Estimated Fair Value, 12 Months or Longer	28	23
Gross Unrealized Losses, 12 Months or Longer	1	1
Estimated Fair Value, Total	128	88
Gross Unrealized Losses, Total	2	2
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	136	52
Gross Unrealized Losses, Less than 12 Months	2	1
Estimated Fair Value, 12 Months or Longer	452	450
Gross Unrealized Losses, 12 Months or Longer	35	33
Estimated Fair Value, Total	588	502
Gross Unrealized Losses, Total	$ 37	$ 34