Variable Interest Entities - Narrative (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,150,000,000	$ 2,270,000,000
Variable interest entity, other short-term borrowings	$ 1,790,000,000	$ 1,870,000,000
Weighted average life of trusts (in years)	5 years 4 months 24 days	5 years 10 months 18 days
Total standby bond-purchase agreement committed to trusts	$ 1,790,000,000
Total letters of credit committed to trusts	598,000,000
Standby purchase agreements and letters of credit commitments utilized	0
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	497,000,000	$ 65,000,000
Liabilities	403,000,000	13,000,000
Potential maximum loss exposure of unconsolidated funds	62,000,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 86,000,000	$ 45,000,000