Variable Interest Entities - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,240,000,000		$ 2,270,000,000
Variable interest entity, other short-term borrowings	1,840,000,000		1,870,000,000
Weighted average life of trusts (in years)	5 years 8 months 15 days	5 years 10 months 18 days
Total standby bond-purchase agreement committed to trusts	1,880,000,000
Total letters of credit committed to trusts	669,000,000
Standby purchase agreements and letters of credit commitments utilized	0
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	68,000,000		65,000,000
Liabilities	14,000,000		13,000,000
Potential maximum loss exposure of unconsolidated funds	54,000,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 88,070,000.00		$ 45,100,000.00