Variable Interest Entities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,680,000,000	$ 2,810,000,000
Variable interest entity, other short-term borrowings	2,150,000,000	2,290,000,000
Weighted average life of trusts (in years)	6 years 11 months 5 days	7 years 4 months 24 days
Total standby bond-purchase agreement committed to trusts	2,190,000,000
Letters of credit amounts outstanding	$ 666,000,000
Number of standby purchase agreements utilized at period end	0