Loan Sale and Servicing Activities and Variable Interest Entities (Narrative) (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Liability related to investments in low income housing tax credits	$ 0.5
Cumulative effect of adopting ASU 2014-01 | Low Income Housing Tax Credit Investments
Other Tax Benefits Recognized Low Income Housing Tax Credit Investments	0.1	$ 0.1
Tax Credits Recognized Low Income Housing Tax Credit Investments	0.2	0.2
Amortization Recognized Low Income Housing Tax Credit Investments	$ 0.2	$ 0.2