Variable Interest Entities (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,300,000,000	$ 2,330,000,000
Variable interest entity, other short-term borrowings	1,890,000,000	1,950,000,000
Weighted average life of trusts (in years)	6 years 1 month	6 years 6 months
Total standby bond-purchase agreement committed to trusts	1,920,000,000
Total letters of credit committed to trusts	684,000,000
Standby purchase agreements and letters of credit commitments utilized	0
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	313,000,000	50,000,000
Liabilities	260,000,000
Potential maximum loss exposure of unconsolidated funds	53,000,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 40,000,000	$ 18,000,000