Variable Interest Entities (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,630,000,000	$ 2,680,000,000
Variable interest entity, other short-term borrowings	2,120,000,000	2,150,000,000
Weighted average life of trusts (in years)	6 years 8 months 12 days	6 years 11 months
Total standby bond-purchase agreement committed to trusts	2,160,000,000
Letters of credit amounts outstanding	$ 666,000,000
Number of standby purchase agreements utilized at period end	0