Investment Securities (Investments, by type and length in continuous loss position) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Domestic Corporate Debt Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	$ 722	$ 434
Gross unrealized losses, less than 12 months	(15)	(7)
Estimated fair value, 12 months or more	414	813
Gross unrealized losses, 12 months or more	(73)	(295)
US States and Political Subdivisions Debt Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	232	146
Gross unrealized losses, less than 12 months	(4)	(2)
Estimated fair value, 12 months or more	327	326
Gross unrealized losses, 12 months or more	(108)	(111)
Residential Mortgage Backed Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	96	98
Gross unrealized losses, less than 12 months	(1)	(1)
Estimated fair value, 12 months or more	663	691
Gross unrealized losses, 12 months or more	(89)	(118)
Commercial Mortgage Backed Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	117	37
Gross unrealized losses, less than 12 months	(1)	0
Estimated fair value, 12 months or more	921	979
Gross unrealized losses, 12 months or more	(77)	(95)
Asset-backed Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	11	18
Gross unrealized losses, less than 12 months	(1)	(1)
Estimated fair value, 12 months or more	568	658
Gross unrealized losses, 12 months or more	(54)	(76)
Foreign Corporate Debt Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	240	167
Gross unrealized losses, less than 12 months	(4)	(8)
Estimated fair value, 12 months or more	606	602
Gross unrealized losses, 12 months or more	(106)	(118)
Foreign Government Debt Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	554	201
Gross unrealized losses, less than 12 months	(1)	(1)
Estimated fair value, 12 months or more	38	37
Gross unrealized losses, 12 months or more	(2)	(2)
US Treasury and Government [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	253	0
Gross unrealized losses, less than 12 months	0	0
Estimated fair value, 12 months or more	0	0
Gross unrealized losses, 12 months or more	0	0
Retained Interest [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	5	3
Gross unrealized losses, less than 12 months	0	0
Estimated fair value, 12 months or more	0	0
Gross unrealized losses, 12 months or more	0	0
Equity Securities [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	22	26
Gross unrealized losses, less than 12 months	(10)	(3)
Estimated fair value, 12 months or more	0	0
Gross unrealized losses, 12 months or more	0	0
Total [Member]
Investment [Line Items]
Estimated fair value, less than 12 months	2,252	1,130
Gross unrealized losses, less than 12 months	(37)	(23)
Estimated fair value, 12 months or more	3,537	4,106
Gross unrealized losses, 12 months or more	$ (509)	$ (815)