Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Investment securities related to state and political subdivisions	$ 1,060	$ 1,050
Variable interest entity, other short-term borrowings	$ 930	$ 930
Weighted average life of trusts	3 years 4 months 24 days	3 years 7 months 6 days
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 37	$ 70