Investments (Securities in a gross unrealized loss position) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Estimated Fair Value, Less Than 12 Months	$ 2,743	$ 1,819
Gross Unrealized Losses, Less than 12 Months	51	27
Estimated Fair Value, 12 Months or Longer	659	866
Gross Unrealized Losses, 12 Months or Longer	107	128
Estimated Fair Value, Total	3,402	2,685
Gross Unrealized Losses, Total	158	155
Corporate and other bonds [Member]
Estimated Fair Value, Less Than 12 Months	951	846
Gross Unrealized Losses, Less than 12 Months	13	13
Estimated Fair Value, 12 Months or Longer	50	108
Gross Unrealized Losses, 12 Months or Longer	5	8
Estimated Fair Value, Total	1,001	954
Gross Unrealized Losses, Total	18	21
States, municipalities and political subdivisions [Member]
Estimated Fair Value, Less Than 12 Months	683	254
Gross Unrealized Losses, Less than 12 Months	16	5
Estimated Fair Value, 12 Months or Longer	121	165
Gross Unrealized Losses, 12 Months or Longer	43	39
Estimated Fair Value, Total	804	419
Gross Unrealized Losses, Total	59	44
Residential mortgage-backed [Member]
Estimated Fair Value, Less Than 12 Months	920	583
Gross Unrealized Losses, Less than 12 Months	19	5
Estimated Fair Value, 12 Months or Longer	347	452
Gross Unrealized Losses, 12 Months or Longer	51	66
Estimated Fair Value, Total	1,267	1,035
Gross Unrealized Losses, Total	70	71
Commercial mortgage-backed [Member]
Estimated Fair Value, Less Than 12 Months	155	85
Gross Unrealized Losses, Less than 12 Months	2	2
Estimated Fair Value, 12 Months or Longer	141	141
Gross Unrealized Losses, 12 Months or Longer	8	15
Estimated Fair Value, Total	296	226
Gross Unrealized Losses, Total	10	17
Total asset-backed [Member]
Estimated Fair Value, Less Than 12 Months	1,075	668
Gross Unrealized Losses, Less than 12 Months	21	7
Estimated Fair Value, 12 Months or Longer	488	593
Gross Unrealized Losses, 12 Months or Longer	59	81
Estimated Fair Value, Total	1,563	1,261
Gross Unrealized Losses, Total	80	88
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	34	28
Gross Unrealized Losses, Less than 12 Months	1	1
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Estimated Fair Value, Total	34	28
Gross Unrealized Losses, Total	$ 1	$ 1