Loans and Allowance for Loan Losses - Accretable Yield for Purchased Credit-Impaired Loans (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Concentrations of Credit Risk
Loans	$ 80,014	$ 77,551
Consumer
Concentrations of Credit Risk
Loans	39,105	33,445
Consumer | Residential mortgage | Loans Receivable
Concentrations of Credit Risk
Loans	35,600
Commercial
Concentrations of Credit Risk
Loans	40,909	$ 44,106
Commercial | Loans Receivable | Financial and Insurance
Concentrations of Credit Risk
Loans	4,700
Additional unfunded commitments to extend credit	5,600
Commercial | Loans Receivable | Power and utilities
Concentrations of Credit Risk
Loans	3,700
Additional unfunded commitments to extend credit	5,300
Commercial | Loans Receivable | Oil and gas
Concentrations of Credit Risk
Loans	1,200
Additional unfunded commitments to extend credit	1,200
Commercial | Loans Receivable | Manufacturing
Concentrations of Credit Risk
Loans	3,900
Additional unfunded commitments to extend credit	3,100
Commercial | Commercial mortgage | Loans Receivable
Concentrations of Credit Risk
Loans	15,200
Additional unfunded commitments to extend credit	$ 4,100