Loans and Allowance for Loan Losses - Accretable Yield for Purchased Credit-Impaired Loans (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Concentrations of Credit Risk
Loans	$ 86,507	$ 80,014
Consumer
Concentrations of Credit Risk
Loans	43,372	39,105
Consumer | Residential mortgage | Notes Receivable [Member]
Concentrations of Credit Risk
Loans	38,000
Commercial
Concentrations of Credit Risk
Loans	43,135	$ 40,909
Commercial | Notes Receivable [Member] | Financial and Insurance [Member]
Concentrations of Credit Risk
Loans	7,000
Additional unfunded commitments to extend credit	7,000
Commercial | Notes Receivable [Member] | Power and Utilities [Member]
Concentrations of Credit Risk
Loans	4,000
Additional unfunded commitments to extend credit	5,000
Commercial | Notes Receivable [Member] | Oil and Gas [Member]
Concentrations of Credit Risk
Loans	3,000
Additional unfunded commitments to extend credit	2,000
Commercial | Notes Receivable [Member] | Manufacturing Sector [Member]
Concentrations of Credit Risk
Loans	4,000
Additional unfunded commitments to extend credit	3,000
Commercial | Commercial mortgage | Notes Receivable [Member]
Concentrations of Credit Risk
Loans	16,000
Additional unfunded commitments to extend credit	$ 4,000