Variable Interest Entities (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,380,000,000	$ 2,680,000,000
Variable interest entity, other short-term borrowings	1,980,000,000	2,150,000,000
Weighted average life of trusts (in years)	6 years 8 months	6 years 11 months
Total standby bond-purchase agreement committed to trusts	2,020,000,000
Letters of credit amounts outstanding	691,000,000
Standby purchase agreements and letters of credit commitments utlized	0
Potential maximum loss exposure of unconsolidated funds	$ 18,000,000	$ 28,000,000