Investments (Securities in a gross unrealized loss position) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 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	$ 3,422	$ 4,275
Gross Unrealized Losses, Less than 12 Months	110	211
Estimated Fair Value, 12 Months or Longer	1,732	1,923
Gross Unrealized Losses, 12 Months or Longer	221	330
Estimated Fair Value, Total	5,154	6,198
Gross Unrealized Losses, Total	331	541
Corporate and other bonds [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	1,635	2,552
Gross Unrealized Losses, Less than 12 Months	43	126
Estimated Fair Value, 12 Months or Longer	124	159
Gross Unrealized Losses, 12 Months or Longer	18	28
Estimated Fair Value, Total	1,759	2,711
Gross Unrealized Losses, Total	61	154
States, municipalities and political subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	385	67
Gross Unrealized Losses, Less than 12 Months	8	1
Estimated Fair Value, 12 Months or Longer	460	721
Gross Unrealized Losses, 12 Months or Longer	85	135
Estimated Fair Value, Total	845	788
Gross Unrealized Losses, Total	93	136
Residential mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	882	719
Gross Unrealized Losses, Less than 12 Months	39	36
Estimated Fair Value, 12 Months or Longer	1,026	874
Gross Unrealized Losses, 12 Months or Longer	100	147
Estimated Fair Value, Total	1,908	1,593
Gross Unrealized Losses, Total	139	183
Commercial mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	219	431
Gross Unrealized Losses, Less than 12 Months	18	39
Estimated Fair Value, 12 Months or Longer	122	169
Gross Unrealized Losses, 12 Months or Longer	18	20
Estimated Fair Value, Total	341	600
Gross Unrealized Losses, Total	36	59
Other asset-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	297	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	297	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	1,398	1,539
Gross Unrealized Losses, Less than 12 Months	58	79
Estimated Fair Value, 12 Months or Longer	1,148	1,043
Gross Unrealized Losses, 12 Months or Longer	118	167
Estimated Fair Value, Total	2,546	2,582
Gross Unrealized Losses, Total	176	246
Total fixed maturity securities available-for-sale [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	3,418	4,158
Gross Unrealized Losses, Less than 12 Months	109	206
Estimated Fair Value, 12 Months or Longer	1,732	1,923
Gross Unrealized Losses, 12 Months or Longer	221	330
Estimated Fair Value, Total	5,150	6,081
Gross Unrealized Losses, Total	330	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
Common stock [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	4
Gross Unrealized Losses, Less than 12 Months	1
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	4
Gross Unrealized Losses, Total	1
Total equity securities available-for-sale [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	4	117
Gross Unrealized Losses, Less than 12 Months	1	5
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	4	117
Gross Unrealized Losses, Total	$ 1	$ 5