Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [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
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 4,229	1,354
Gross Unrealized Losses, Less than 12 Months	104	21
Estimated Fair Value, 12 Months or Longer	146	168
Gross Unrealized Losses, 12 Months or Longer	11	7
Estimated Fair Value, Total	4,375	1,522
Gross Unrealized Losses, Total	115	28
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	962	72
Gross Unrealized Losses, Less than 12 Months	12	1
Estimated Fair Value, 12 Months or Longer	3	85
Gross Unrealized Losses, 12 Months or Longer	0	1
Estimated Fair Value, Total	965	157
Gross Unrealized Losses, Total	12	2
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2,570	1,228
Gross Unrealized Losses, Less than 12 Months	55	5
Estimated Fair Value, 12 Months or Longer	504	947
Gross Unrealized Losses, 12 Months or Longer	23	27
Estimated Fair Value, Total	3,074	2,175
Gross Unrealized Losses, Total	78	32
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	668	403
Gross Unrealized Losses, Less than 12 Months	11	4
Estimated Fair Value, 12 Months or Longer	200	212
Gross Unrealized Losses, 12 Months or Longer	11	10
Estimated Fair Value, Total	868	615
Gross Unrealized Losses, Total	22	14
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	441	248
Gross Unrealized Losses, Less than 12 Months	5	3
Estimated Fair Value, 12 Months or Longer	15	18
Gross Unrealized Losses, 12 Months or Longer	2	2
Estimated Fair Value, Total	456	266
Gross Unrealized Losses, Total	7	5
Total asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,679	1,879
Gross Unrealized Losses, Less than 12 Months	71	12
Estimated Fair Value, 12 Months or Longer	719	1,177
Gross Unrealized Losses, 12 Months or Longer	36	39
Estimated Fair Value, Total	4,398	3,056
Gross Unrealized Losses, Total	107	51
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	59	49
Gross Unrealized Losses, Less than 12 Months	4	2
Estimated Fair Value, 12 Months or Longer	26	21
Gross Unrealized Losses, 12 Months or Longer	3	2
Estimated Fair Value, Total	85	70
Gross Unrealized Losses, Total	7	4
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	214	166
Gross Unrealized Losses, Less than 12 Months	5	2
Estimated Fair Value, 12 Months or Longer	4	4
Gross Unrealized Losses, 12 Months or Longer	0	0
Estimated Fair Value, Total	218	170
Gross Unrealized Losses, Total	5	2
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	9,143	3,520
Gross Unrealized Losses, Less than 12 Months	196	38
Estimated Fair Value, 12 Months or Longer	898	1,455
Gross Unrealized Losses, 12 Months or Longer	50	49
Estimated Fair Value, Total	10,041	4,975
Gross Unrealized Losses, Total	$ 246	87
Common Stock
Schedule of Available-for-sale Securities [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
Schedule of Available-for-sale Securities [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
Schedule of Available-for-sale Securities [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