Variable Interest Entities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Investment securities related to state and political subdivisions	$ 2,100,000,000	$ 1,350,000,000
Variable interest entity, other short-term borrowings	$ 1,750,000,000	1,160,000,000
Weighted average life of trusts	5 years 5 months 10 days
Total standby bond-purchase agreement committed to trusts		1,160,000,000
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	$ 321,000,000	0
Liabilities	228,000,000	0
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 93,000,000	$ 121,000,000