Pledged Assets (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 39,700,000,000		$ 42,100,000,000
Securities eligible to be repledged or sold	31,400,000		487,200,000
Amount of securities re pledged or sold collateral	0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,700,000,000	$ 1,500,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 7,500,000,000		$ 8,500,000,000