Variable Interest Entities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Investment securities related to state and political subdivisions	$ 1,210	$ 1,250
Variable interest entity, other short-term borrowings	$ 1,060	$ 1,080
Weighted average life of trusts	3 years 7 months 9 days	4 years 7 months
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	$ 188	$ 149
Liabilities	88	50
Potential maximum loss exposure of unconsolidated funds	100
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 72	$ 72