Investments (Securities in a gross unrealized loss position) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Total available-for-sale securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	$ 2,339	$ 4,275
Gross Unrealized Losses, Less than 12 Months	74	211
Estimated Fair Value, 12 Months or Longer	1,569	1,923
Gross Unrealized Losses, 12 Months or Longer	240	330
Estimated Fair Value, Total	3,908	6,198
Gross Unrealized Losses, Total	314	541
Corporate and other bonds [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	1,550	2,552
Gross Unrealized Losses, Less than 12 Months	53	126
Estimated Fair Value, 12 Months or Longer	192	159
Gross Unrealized Losses, 12 Months or Longer	26	28
Estimated Fair Value, Total	1,742	2,711
Gross Unrealized Losses, Total	79	154
States, municipalities and political subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	174	67
Gross Unrealized Losses, Less than 12 Months	2	1
Estimated Fair Value, 12 Months or Longer	301	721
Gross Unrealized Losses, 12 Months or Longer	64	135
Estimated Fair Value, Total	475	788
Gross Unrealized Losses, Total	66	136
Residential mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	276	719
Gross Unrealized Losses, Less than 12 Months	13	36
Estimated Fair Value, 12 Months or Longer	923	874
Gross Unrealized Losses, 12 Months or Longer	128	147
Estimated Fair Value, Total	1,199	1,593
Gross Unrealized Losses, Total	141	183
Commercial mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	158	431
Gross Unrealized Losses, Less than 12 Months	5	39
Estimated Fair Value, 12 Months or Longer	153	169
Gross Unrealized Losses, 12 Months or Longer	22	20
Estimated Fair Value, Total	311	600
Gross Unrealized Losses, Total	27	59
Other asset-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	181	389
Gross Unrealized Losses, Less than 12 Months	1	4
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	181	389
Gross Unrealized Losses, Total	1	4
Total asset-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	615	1,539
Gross Unrealized Losses, Less than 12 Months	19	79
Estimated Fair Value, 12 Months or Longer	1,076	1,043
Gross Unrealized Losses, 12 Months or Longer	150	167
Estimated Fair Value, Total	1,691	2,582
Gross Unrealized Losses, Total	169	246
Total fixed maturity securities available-for-sale [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	2,339	4,158
Gross Unrealized Losses, Less than 12 Months	74	206
Estimated Fair Value, 12 Months or Longer	1,569	1,923
Gross Unrealized Losses, 12 Months or Longer	240	330
Estimated Fair Value, Total	3,908	6,081
Gross Unrealized Losses, Total	314	536
Preferred stock [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months		117
Gross Unrealized Losses, Less than 12 Months		5
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total		117
Gross Unrealized Losses, Total		$ 5