Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		$ 3,620
Gross Unrealized Losses, Less than 12 Months		40
Estimated Fair Value, 12 Months or Longer		1,455
Gross Unrealized Losses, 12 Months or Longer		49
Estimated Fair Value, Total		5,075
Gross Unrealized Losses, Total		89
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	$ 6,963
Gross Unrealized Losses, Less than 12 Months	192
Estimated Fair Value, 12 Months or Longer	193
Gross Unrealized Losses, 12 Months or Longer	11
Estimated Fair Value, Total	7,156
Gross Unrealized Losses, Total	203
Estimated Fair Value, Less than 12 Months		1,354
Gross Unrealized Losses, Less than 12 Months		21
Estimated Fair Value, 12 Months or Longer		168
Gross Unrealized Losses, 12 Months or Longer		7
Estimated Fair Value, Total		1,522
Gross Unrealized Losses, Total		28
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	723
Gross Unrealized Losses, Less than 12 Months	8
Estimated Fair Value, 12 Months or Longer	3
Gross Unrealized Losses, 12 Months or Longer	0
Estimated Fair Value, Total	726
Gross Unrealized Losses, Total	8
Estimated Fair Value, Less than 12 Months		72
Gross Unrealized Losses, Less than 12 Months		1
Estimated Fair Value, 12 Months or Longer		85
Gross Unrealized Losses, 12 Months or Longer		1
Estimated Fair Value, Total		157
Gross Unrealized Losses, Total		2
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	3,183
Gross Unrealized Losses, Less than 12 Months	68
Estimated Fair Value, 12 Months or Longer	344
Gross Unrealized Losses, 12 Months or Longer	17
Estimated Fair Value, Total	3,527
Gross Unrealized Losses, Total	85
Estimated Fair Value, Less than 12 Months		1,228
Gross Unrealized Losses, Less than 12 Months		5
Estimated Fair Value, 12 Months or Longer		947
Gross Unrealized Losses, 12 Months or Longer		27
Estimated Fair Value, Total		2,175
Gross Unrealized Losses, Total		32
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	913
Gross Unrealized Losses, Less than 12 Months	16
Estimated Fair Value, 12 Months or Longer	228
Gross Unrealized Losses, 12 Months or Longer	17
Estimated Fair Value, Total	1,141
Gross Unrealized Losses, Total	33
Estimated Fair Value, Less than 12 Months		403
Gross Unrealized Losses, Less than 12 Months		4
Estimated Fair Value, 12 Months or Longer		212
Gross Unrealized Losses, 12 Months or Longer		10
Estimated Fair Value, Total		615
Gross Unrealized Losses, Total		14
Other asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	632
Gross Unrealized Losses, Less than 12 Months	6
Estimated Fair Value, 12 Months or Longer	27
Gross Unrealized Losses, 12 Months or Longer	3
Estimated Fair Value, Total	659
Gross Unrealized Losses, Total	9
Estimated Fair Value, Less than 12 Months		248
Gross Unrealized Losses, Less than 12 Months		3
Estimated Fair Value, 12 Months or Longer		18
Gross Unrealized Losses, 12 Months or Longer		2
Estimated Fair Value, Total		266
Gross Unrealized Losses, Total		5
Total asset-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	4,728
Gross Unrealized Losses, Less than 12 Months	90
Estimated Fair Value, 12 Months or Longer	599
Gross Unrealized Losses, 12 Months or Longer	37
Estimated Fair Value, Total	5,327
Gross Unrealized Losses, Total	127
Estimated Fair Value, Less than 12 Months		1,879
Gross Unrealized Losses, Less than 12 Months		12
Estimated Fair Value, 12 Months or Longer		1,177
Gross Unrealized Losses, 12 Months or Longer		39
Estimated Fair Value, Total		3,056
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	49
Gross Unrealized Losses, Less than 12 Months	1
Estimated Fair Value, 12 Months or Longer	19
Gross Unrealized Losses, 12 Months or Longer	2
Estimated Fair Value, Total	68
Gross Unrealized Losses, Total	3
Estimated Fair Value, Less than 12 Months		49
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer		21
Gross Unrealized Losses, 12 Months or Longer		2
Estimated Fair Value, Total		70
Gross Unrealized Losses, Total		4
Foreign government
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	178
Gross Unrealized Losses, Less than 12 Months	3
Estimated Fair Value, 12 Months or Longer	40
Gross Unrealized Losses, 12 Months or Longer	2
Estimated Fair Value, Total	218
Gross Unrealized Losses, Total	5
Estimated Fair Value, Less than 12 Months		166
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer		4
Gross Unrealized Losses, 12 Months or Longer		0
Estimated Fair Value, Total		170
Gross Unrealized Losses, Total		2
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	12,641
Gross Unrealized Losses, Less than 12 Months	294
Estimated Fair Value, 12 Months or Longer	854
Gross Unrealized Losses, 12 Months or Longer	52
Estimated Fair Value, Total	13,495
Gross Unrealized Losses, Total	$ 346
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