Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	$ 12,689
Estimated Fair Value, Less than 12 Months		$ 3,620
Gross Unrealized Losses, Less than 12 Months	404
Gross Unrealized Losses, Less than 12 Months		40
Estimated Fair Value, 12 Months or Longer	2,614
Estimated Fair Value, 12 Months or Longer		1,455
Gross Unrealized Losses, 12 Months or Longer	117
Gross Unrealized Losses, 12 Months or Longer		49
Estimated Fair Value, Total	15,303
Estimated Fair Value, Total		5,075
Gross Unrealized Losses, Total	521
Gross Unrealized Losses, Total		89
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	8,543
Estimated Fair Value, Less than 12 Months		1,354
Gross Unrealized Losses, Less than 12 Months	340
Gross Unrealized Losses, Less than 12 Months		21
Estimated Fair Value, 12 Months or Longer	825
Estimated Fair Value, 12 Months or Longer		168
Gross Unrealized Losses, 12 Months or Longer	55
Gross Unrealized Losses, 12 Months or Longer		7
Estimated Fair Value, Total	9,368
Estimated Fair Value, Total		1,522
Gross Unrealized Losses, Total	395
Gross Unrealized Losses, Total		28
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	517
Estimated Fair Value, Less than 12 Months		72
Gross Unrealized Losses, Less than 12 Months	8
Gross Unrealized Losses, Less than 12 Months		1
Estimated Fair Value, 12 Months or Longer	5
Estimated Fair Value, 12 Months or Longer		85
Gross Unrealized Losses, 12 Months or Longer	1
Gross Unrealized Losses, 12 Months or Longer		1
Estimated Fair Value, Total	522
Estimated Fair Value, Total		157
Gross Unrealized Losses, Total	9
Gross Unrealized Losses, Total		2
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	1,932
Estimated Fair Value, Less than 12 Months		1,228
Gross Unrealized Losses, Less than 12 Months	23
Gross Unrealized Losses, Less than 12 Months		5
Estimated Fair Value, 12 Months or Longer	1,119
Estimated Fair Value, 12 Months or Longer		947
Gross Unrealized Losses, 12 Months or Longer	34
Gross Unrealized Losses, 12 Months or Longer		27
Estimated Fair Value, Total	3,051
Estimated Fair Value, Total		2,175
Gross Unrealized Losses, Total	57
Gross Unrealized Losses, Total		32
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	728
Estimated Fair Value, Less than 12 Months		403
Gross Unrealized Losses, Less than 12 Months	10
Gross Unrealized Losses, Less than 12 Months		4
Estimated Fair Value, 12 Months or Longer	397
Estimated Fair Value, 12 Months or Longer		212
Gross Unrealized Losses, 12 Months or Longer	22
Gross Unrealized Losses, 12 Months or Longer		10
Estimated Fair Value, Total	1,125
Estimated Fair Value, Total		615
Gross Unrealized Losses, Total	32
Gross Unrealized Losses, Total		14
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	834
Estimated Fair Value, Less than 12 Months		248
Gross Unrealized Losses, Less than 12 Months	21
Gross Unrealized Losses, Less than 12 Months		3
Estimated Fair Value, 12 Months or Longer	125
Estimated Fair Value, 12 Months or Longer		18
Gross Unrealized Losses, 12 Months or Longer	3
Gross Unrealized Losses, 12 Months or Longer		2
Estimated Fair Value, Total	959
Estimated Fair Value, Total		266
Gross Unrealized Losses, Total	24
Gross Unrealized Losses, Total		5
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	3,494
Estimated Fair Value, Less than 12 Months		1,879
Gross Unrealized Losses, Less than 12 Months	54
Gross Unrealized Losses, Less than 12 Months		12
Estimated Fair Value, 12 Months or Longer	1,641
Estimated Fair Value, 12 Months or Longer		1,177
Gross Unrealized Losses, 12 Months or Longer	59
Gross Unrealized Losses, 12 Months or Longer		39
Estimated Fair Value, Total	5,135
Estimated Fair Value, Total		3,056
Gross Unrealized Losses, Total	113
Gross Unrealized Losses, Total		51
U.S. Treasury and obligations of government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	21
Estimated Fair Value, Less than 12 Months		49
Gross Unrealized Losses, Less than 12 Months	0
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer	19
Estimated Fair Value, 12 Months or Longer		21
Gross Unrealized Losses, 12 Months or Longer	0
Gross Unrealized Losses, 12 Months or Longer		2
Estimated Fair Value, Total	40
Estimated Fair Value, Total		70
Gross Unrealized Losses, Total	0
Gross Unrealized Losses, Total		4
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 months	114
Estimated Fair Value, Less than 12 Months		166
Gross Unrealized Losses, Less than 12 Months	2
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer	124
Estimated Fair Value, 12 Months or Longer		4
Gross Unrealized Losses, 12 Months or Longer	2
Gross Unrealized Losses, 12 Months or Longer		0
Estimated Fair Value, Total	238
Estimated Fair Value, Total		170
Gross Unrealized Losses, Total	$ 4
Gross Unrealized Losses, Total		2
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		3,520
Gross Unrealized Losses, Less than 12 Months		38
Estimated Fair Value, 12 Months or Longer		1,455
Gross Unrealized Losses, 12 Months or Longer		49
Estimated Fair Value, Total		4,975
Gross Unrealized Losses, Total		87
Common Stock
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		7
Gross Unrealized Losses, Less than 12 Months		1
Estimated Fair Value, 12 Months or Longer		0
Gross Unrealized Losses, 12 Months or Longer		0
Estimated Fair Value, Total		7
Gross Unrealized Losses, Total		1
Preferred stock
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		93
Gross Unrealized Losses, Less than 12 Months		1
Estimated Fair Value, 12 Months or Longer		0
Gross Unrealized Losses, 12 Months or Longer		0
Estimated Fair Value, Total		93
Gross Unrealized Losses, Total		1
Equity securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		100
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer		0
Gross Unrealized Losses, 12 Months or Longer		0
Estimated Fair Value, Total		100
Gross Unrealized Losses, Total		$ 2