Investments (Securities in a gross unrealized loss position) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Total available-for-sale securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	$ 4,275	$ 7,327
Gross Unrealized Losses, Less than 12 Months	211	260
Estimated Fair Value, 12 Months or Longer	1,923	3,414
Gross Unrealized Losses, 12 Months or Longer	330	544
Estimated Fair Value, Total	6,198	10,741
Gross Unrealized Losses, Total	541	804
Corporate and other bonds [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	2,552	1,719
Gross Unrealized Losses, Less than 12 Months	126	34
Estimated Fair Value, 12 Months or Longer	159	405
Gross Unrealized Losses, 12 Months or Longer	28	36
Estimated Fair Value, Total	2,711	2,124
Gross Unrealized Losses, Total	154	70
States, municipalities and political subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	67	3,339
Gross Unrealized Losses, Less than 12 Months	1	164
Estimated Fair Value, 12 Months or Longer	721	745
Gross Unrealized Losses, 12 Months or Longer	135	246
Estimated Fair Value, Total	788	4,084
Gross Unrealized Losses, Total	136	410
Residential mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	719	1,800
Gross Unrealized Losses, Less than 12 Months	36	52
Estimated Fair Value, 12 Months or Longer	874	1,801
Gross Unrealized Losses, 12 Months or Longer	147	213
Estimated Fair Value, Total	1,593	3,601
Gross Unrealized Losses, Total	183	265
Commercial mortgage-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	431	164
Gross Unrealized Losses, Less than 12 Months	39	3
Estimated Fair Value, 12 Months or Longer	169	333
Gross Unrealized Losses, 12 Months or Longer	20	38
Estimated Fair Value, Total	600	497
Gross Unrealized Losses, Total	59	41
Other asset-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	389	122
Gross Unrealized Losses, Less than 12 Months	4	1
Estimated Fair Value, 12 Months or Longer		60
Gross Unrealized Losses, 12 Months or Longer		7
Estimated Fair Value, Total	389	182
Gross Unrealized Losses, Total	4	8
Total asset-backed [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	1,539	2,086
Gross Unrealized Losses, Less than 12 Months	79	56
Estimated Fair Value, 12 Months or Longer	1,043	2,194
Gross Unrealized Losses, 12 Months or Longer	167	258
Estimated Fair Value, Total	2,582	4,280
Gross Unrealized Losses, Total	246	314
US Treasury and Government [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months		8
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		8
Gross Unrealized Losses, Total		1
Total fixed maturity securities available-for-sale [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	4,158	7,152
Gross Unrealized Losses, Less than 12 Months	206	255
Estimated Fair Value, 12 Months or Longer	1,923	3,344
Gross Unrealized Losses, 12 Months or Longer	330	540
Estimated Fair Value, Total	6,081	10,496
Gross Unrealized Losses, Total	536	795
Preferred stock [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	117	175
Gross Unrealized Losses, Less than 12 Months	5	5
Estimated Fair Value, 12 Months or Longer		70
Gross Unrealized Losses, 12 Months or Longer		4
Estimated Fair Value, Total	117	245
Gross Unrealized Losses, Total	5	9
Total equity securities available-for-sale [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position
Estimated Fair Value, Less than 12 Months	117	175
Gross Unrealized Losses, Less than 12 Months	5	5
Estimated Fair Value, 12 Months or Longer		70
Gross Unrealized Losses, 12 Months or Longer		4
Estimated Fair Value, Total	117	245
Gross Unrealized Losses, Total	$ 5	$ 9