Variable Interest Entities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,100,000,000	$ 2,270,000,000
Variable interest entity, other short-term borrowings	$ 1,750,000,000	$ 1,870,000,000
Weighted average life of trusts	5 years 4 months 24 days	5 years 10 months 18 days
Total standby bond-purchase agreement committed to trusts	$ 1,750,000,000
Total letters of credit committed to trusts	569,000,000
Standby purchase agreements and letters of credit commitments utilized	0
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	321,000,000	$ 65,000,000
Liabilities	228,000,000	13,000,000
Potential maximum loss exposure of unconsolidated funds	61,000,000
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 75,000,000	$ 45,000,000