Variable Interest Entities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Investment securities related to state and political subdivisions	$ 1,760	$ 2,100
Variable interest entity, other short-term borrowings	$ 1,390	$ 1,750
Weighted average life of trusts	4 years 9 months 20 days	5 years 5 months 10 days
Total standby bond-purchase agreement committed to trusts	$ 1,620
Total letters of credit committed to trusts	530
Standby purchase agreements and letters of credit commitments utilized	81
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	409	$ 321
Liabilities	327	228
Potential maximum loss exposure of unconsolidated funds	51
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 92	$ 75