Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified			3 Months Ended
	Mar. 31, 2013 Level 2 Financial Products Nonrecurring basis	Dec. 31, 2012 Level 2 Financial Products Nonrecurring basis	Mar. 31, 2013 Level 3 Guarantees Recurring basis	Mar. 31, 2012 Level 3 Guarantees Recurring basis
Roll-forward of liabilities measured at fair value using Level 3 inputs
Balance at beginning of year			$ 14	$ 7
Issuance of guarantees			5	0
Expiration of guarantees			(3)	0
Balance at end of period			16	7
Fair value of impaired loans
Impaired loans	$ 190	$ 161