Variable Interest Entities - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Variable Interest Entities [Abstract]
Investment securities related to state and political subdivisions	$ 2,060,000,000	$ 2,100,000,000
Variable interest entity, other short-term borrowings	$ 1,680,000,000	$ 1,750,000,000
Weighted average life of trusts	5 years 3 months 10 days	5 years 5 months 10 days
Total standby bond-purchase agreement committed to trusts	$ 1,710,000,000
Total letters of credit committed to trusts	546,000,000
Standby purchase agreements and letters of credit commitments utilized	20,000,000
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	330,000,000	$ 321,000,000
Liabilities	241,000,000	228,000,000
Potential maximum loss exposure of unconsolidated funds	57,000,000
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 129,000,000	$ 75,000,000