Certain Transfers of Financial Assets and Variable Interest Entities Tax Credit Variable Interest Entities (Additional Information) (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - Limited Partner [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Community Development Investments [Member]
Portion of Other Assets Not Qualified for Tax Credits	$ 64	$ 59
Loans Issued by the Company to the Limited Partnerships	386
Unfunded Equity Commitment	628	627
Renewable Energy Program [Member]
Unfunded Equity Commitment	$ 179	0
Community Development Investments [Member]
Loans Issued by the Company to the Limited Partnerships		$ 354