Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	$ 3,001	$ 2,148
Gross Unrealized Losses, Less than 12 Months	176	31
Estimated Fair Value, 12 Months or Longer	101	272
Gross Unrealized Losses, 12 Months or Longer	5	14
Estimated Fair Value, Total	3,102	2,420
Gross Unrealized Losses, Total	181	45
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,579	914
Gross Unrealized Losses, Less than 12 Months	86	21
Estimated Fair Value, 12 Months or Longer	56	186
Gross Unrealized Losses, 12 Months or Longer	3	11
Estimated Fair Value, Total	1,635	1,100
Gross Unrealized Losses, Total	89	32
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	154	34
Gross Unrealized Losses, Less than 12 Months	1	0
Estimated Fair Value, 12 Months or Longer	0	0
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	154	34
Gross Unrealized Losses, Total	1
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	98	249
Gross Unrealized Losses, Less than 12 Months	1	1
Estimated Fair Value, 12 Months or Longer	13	30
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	111	279
Gross Unrealized Losses, Total	1	1
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	693	381
Gross Unrealized Losses, Less than 12 Months	69	3
Estimated Fair Value, 12 Months or Longer	19	20
Gross Unrealized Losses, 12 Months or Longer	1	2
Estimated Fair Value, Total	712	401
Gross Unrealized Losses, Total	70	5
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	432	449
Gross Unrealized Losses, Less than 12 Months	18	3
Estimated Fair Value, 12 Months or Longer	13	33
Gross Unrealized Losses, 12 Months or Longer	1	1
Estimated Fair Value, Total	445	482
Gross Unrealized Losses, Total	19	4
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,223	1,079
Gross Unrealized Losses, Less than 12 Months	88	7
Estimated Fair Value, 12 Months or Longer	45	83
Gross Unrealized Losses, 12 Months or Longer	2	3
Estimated Fair Value, Total	1,268	1,162
Gross Unrealized Losses, Total	90	10
U.S. Treasury and obligations of government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	25	62
Gross Unrealized Losses, Less than 12 Months	1	2
Estimated Fair Value, 12 Months or Longer	0	2
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	25	64
Gross Unrealized Losses, Total	1	2
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	20	59
Gross Unrealized Losses, Less than 12 Months	0	1
Estimated Fair Value, 12 Months or Longer	0	1
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	20	60
Gross Unrealized Losses, Total	$ 0	$ 1