Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	$ 2,478	$ 12,689
Gross Unrealized Losses, Less than 12 Months	53	404
Estimated Fair Value, 12 Months or Longer	3,612	2,614
Gross Unrealized Losses, 12 Months or Longer	87	117
Estimated Fair Value, Total	6,090	15,303
Gross Unrealized Losses, Total	140	521
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,656	8,543
Gross Unrealized Losses, Less than 12 Months	39	340
Estimated Fair Value, 12 Months or Longer	1,743	825
Gross Unrealized Losses, 12 Months or Longer	60	55
Estimated Fair Value, Total	3,399	9,368
Gross Unrealized Losses, Total	99	395
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	14	517
Gross Unrealized Losses, Less than 12 Months	0	8
Estimated Fair Value, 12 Months or Longer	3	5
Gross Unrealized Losses, 12 Months or Longer	0	1
Estimated Fair Value, Total	17	522
Gross Unrealized Losses, Total		9
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	94	1,932
Gross Unrealized Losses, Less than 12 Months	0	23
Estimated Fair Value, 12 Months or Longer	1,494	1,119
Gross Unrealized Losses, 12 Months or Longer	20	34
Estimated Fair Value, Total	1,588	3,051
Gross Unrealized Losses, Total	20	57
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	184	728
Gross Unrealized Losses, Less than 12 Months	2	10
Estimated Fair Value, 12 Months or Longer	236	397
Gross Unrealized Losses, 12 Months or Longer	5	22
Estimated Fair Value, Total	420	1,125
Gross Unrealized Losses, Total	7	32
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	450	834
Gross Unrealized Losses, Less than 12 Months	10	21
Estimated Fair Value, 12 Months or Longer	95	125
Gross Unrealized Losses, 12 Months or Longer	2	3
Estimated Fair Value, Total	545	959
Gross Unrealized Losses, Total	12	24
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	728	3,494
Gross Unrealized Losses, Less than 12 Months	12	54
Estimated Fair Value, 12 Months or Longer	1,825	1,641
Gross Unrealized Losses, 12 Months or Longer	27	59
Estimated Fair Value, Total	2,553	5,135
Gross Unrealized Losses, Total	39	113
U.S. Treasury and obligations of government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	48	21
Gross Unrealized Losses, Less than 12 Months	1	0
Estimated Fair Value, 12 Months or Longer	14	19
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	62	40
Gross Unrealized Losses, Total	1	0
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	32	114
Gross Unrealized Losses, Less than 12 Months	1	2
Estimated Fair Value, 12 Months or Longer	27	124
Gross Unrealized Losses, 12 Months or Longer	0	2
Estimated Fair Value, Total	59	238
Gross Unrealized Losses, Total	$ 1	$ 4