Investments (Securities in a gross unrealized loss position) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Estimated Fair Value, Less Than 12 Months	$ 7,947	$ 1,819
Gross Unrealized Losses, Less than 12 Months	334	27
Estimated Fair Value, 12 Months or Longer	552	866
Gross Unrealized Losses, 12 Months or Longer	106	128
Estimated Fair Value, Total	8,499	2,685
Gross Unrealized Losses, Total	440	155
Corporate and other bonds [Member]
Estimated Fair Value, Less Than 12 Months	3,478	846
Gross Unrealized Losses, Less than 12 Months	143	13
Estimated Fair Value, 12 Months or Longer	33	108
Gross Unrealized Losses, 12 Months or Longer	4	8
Estimated Fair Value, Total	3,511	954
Gross Unrealized Losses, Total	147	21
States, municipalities and political subdivisions [Member]
Estimated Fair Value, Less Than 12 Months	2,075	254
Gross Unrealized Losses, Less than 12 Months	130	5
Estimated Fair Value, 12 Months or Longer	119	165
Gross Unrealized Losses, 12 Months or Longer	43	39
Estimated Fair Value, Total	2,194	419
Gross Unrealized Losses, Total	173	44
Residential mortgage-backed [Member]
Estimated Fair Value, Less Than 12 Months	1,462	583
Gross Unrealized Losses, Less than 12 Months	33	5
Estimated Fair Value, 12 Months or Longer	321	452
Gross Unrealized Losses, 12 Months or Longer	55	66
Estimated Fair Value, Total	1,783	1,035
Gross Unrealized Losses, Total	88	71
Commercial mortgage-backed [Member]
Estimated Fair Value, Less Than 12 Months	593	85
Gross Unrealized Losses, Less than 12 Months	23	2
Estimated Fair Value, 12 Months or Longer	79	141
Gross Unrealized Losses, 12 Months or Longer	4	15
Estimated Fair Value, Total	672	226
Gross Unrealized Losses, Total	27	17
Other asset-backed [Member]
Estimated Fair Value, Less Than 12 Months	235
Gross Unrealized Losses, Less than 12 Months	2
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	235
Gross Unrealized Losses, Total	2
Total asset-backed [Member]
Estimated Fair Value, Less Than 12 Months	2,290	668
Gross Unrealized Losses, Less than 12 Months	58	7
Estimated Fair Value, 12 Months or Longer	400	593
Gross Unrealized Losses, 12 Months or Longer	59	81
Estimated Fair Value, Total	2,690	1,261
Gross Unrealized Losses, Total	117	88
Foreign government [Member]
Estimated Fair Value, Less Than 12 Months	65
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	65
Gross Unrealized Losses, Total	1
U.S. Treasury and obligations of government-sponsored enterprises [Member]
Estimated Fair Value, Less Than 12 Months		23
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		23
Gross Unrealized Losses, Total		1
Redeemable preferred stock [Member]
Estimated Fair Value, Less Than 12 Months	39	28
Gross Unrealized Losses, Less than 12 Months	2	1
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	39	28
Gross Unrealized Losses, Total	$ 2	$ 1