Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Sep. 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	$ 7,546
Gross Unrealized Losses, Less than 12 Months	197
Estimated Fair Value, 12 Months or Longer	342
Gross Unrealized Losses, 12 Months or Longer	18
Estimated Fair Value, Total	7,888
Gross Unrealized Losses, Total	215
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	807
Gross Unrealized Losses, Less than 12 Months	19
Estimated Fair Value, 12 Months or Longer	2
Gross Unrealized Losses, 12 Months or Longer	1
Estimated Fair Value, Total	809
Gross Unrealized Losses, Total	20
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	3,409
Gross Unrealized Losses, Less than 12 Months	89
Estimated Fair Value, 12 Months or Longer	501
Gross Unrealized Losses, 12 Months or Longer	26
Estimated Fair Value, Total	3,910
Gross Unrealized Losses, Total	115
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	968
Gross Unrealized Losses, Less than 12 Months	15
Estimated Fair Value, 12 Months or Longer	322
Gross Unrealized Losses, 12 Months or Longer	21
Estimated Fair Value, Total	1,290
Gross Unrealized Losses, Total	36
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	953
Gross Unrealized Losses, Less than 12 Months	9
Estimated Fair Value, 12 Months or Longer	45
Gross Unrealized Losses, 12 Months or Longer	0
Estimated Fair Value, Total	998
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	5,330
Gross Unrealized Losses, Less than 12 Months	113
Estimated Fair Value, 12 Months or Longer	868
Gross Unrealized Losses, 12 Months or Longer	47
Estimated Fair Value, Total	6,198
Gross Unrealized Losses, Total	160
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	35
Gross Unrealized Losses, Less than 12 Months	0
Estimated Fair Value, 12 Months or Longer	42
Gross Unrealized Losses, 12 Months or Longer	2
Estimated Fair Value, Total	77
Gross Unrealized Losses, Total	2
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	188
Gross Unrealized Losses, Less than 12 Months	4
Estimated Fair Value, 12 Months or Longer	64
Gross Unrealized Losses, 12 Months or Longer	2
Estimated Fair Value, Total	252
Gross Unrealized Losses, Total	6
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	13,906
Gross Unrealized Losses, Less than 12 Months	333
Estimated Fair Value, 12 Months or Longer	1,318
Gross Unrealized Losses, 12 Months or Longer	70
Estimated Fair Value, Total	15,224
Gross Unrealized Losses, Total	$ 403
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