Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	$ 10,066	$ 2,148
Gross Unrealized Losses, Less than 12 Months	1,099	31
Estimated Fair Value, 12 Months or Longer	87	272
Gross Unrealized Losses, 12 Months or Longer	18	14
Estimated Fair Value, Total	10,153	2,420
Gross Unrealized Losses, Total	1,117	45
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	7,036	914
Gross Unrealized Losses, Less than 12 Months	804	21
Estimated Fair Value, 12 Months or Longer	44	186
Gross Unrealized Losses, 12 Months or Longer	13	11
Estimated Fair Value, Total	7,080	1,100
Gross Unrealized Losses, Total	817	32
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	82	34
Gross Unrealized Losses, Less than 12 Months	2	0
Estimated Fair Value, 12 Months or Longer	0	0
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	82	34
Gross Unrealized Losses, Total	2
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	166	249
Gross Unrealized Losses, Less than 12 Months	7	1
Estimated Fair Value, 12 Months or Longer	22	30
Gross Unrealized Losses, 12 Months or Longer	1	0
Estimated Fair Value, Total	188	279
Gross Unrealized Losses, Total	8	1
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,185	381
Gross Unrealized Losses, Less than 12 Months	151	3
Estimated Fair Value, 12 Months or Longer	12	20
Gross Unrealized Losses, 12 Months or Longer	2	2
Estimated Fair Value, Total	1,197	401
Gross Unrealized Losses, Total	153	5
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,510	449
Gross Unrealized Losses, Less than 12 Months	131	3
Estimated Fair Value, 12 Months or Longer	9	33
Gross Unrealized Losses, 12 Months or Longer	2	1
Estimated Fair Value, Total	1,519	482
Gross Unrealized Losses, Total	133	4
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	2,861	1,079
Gross Unrealized Losses, Less than 12 Months	289	7
Estimated Fair Value, 12 Months or Longer	43	83
Gross Unrealized Losses, 12 Months or Longer	5	3
Estimated Fair Value, Total	2,904	1,162
Gross Unrealized Losses, Total	294	10
U.S. Treasury and obligations of government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	2	62
Gross Unrealized Losses, Less than 12 Months	0	2
Estimated Fair Value, 12 Months or Longer	0	2
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	2	64
Gross Unrealized Losses, Total	0	2
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	76	59
Gross Unrealized Losses, Less than 12 Months	4	1
Estimated Fair Value, 12 Months or Longer	0	1
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	76	60
Gross Unrealized Losses, Total	4	$ 1
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	9
Gross Unrealized Losses, Less than 12 Months	0
Estimated Fair Value, 12 Months or Longer	0
Gross Unrealized Losses, 12 Months or Longer	0
Estimated Fair Value, Total	9
Gross Unrealized Losses, Total	$ 0