Variable Interest Entities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Investment securities related to state and political subdivisions	$ 1,360,000,000	$ 1,350,000,000
Variable interest entity, other short-term borrowings	$ 1,160,000,000	$ 1,160,000,000
Weighted average life of trusts	4 years 4 months 24 days	4 years 6 months
Total standby bond-purchase agreement committed to trusts	$ 1,180,000,000
Total letters of credit committed to trusts	351,000,000
Standby purchase agreements and letters of credit commitments utilized	0
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	0
Liabilities	0
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 120,000,000	$ 121,000,000