Loans and Allowance for Loan Losses - Accretable Yield for Purchased Credit-Impaired Loans (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Concentrations of Credit Risk
Loans	$ 88,213	$ 86,507
Consumer | Residential mortgage | Notes Receivable [Member]
Concentrations of Credit Risk
Loans	36,000
Commercial | Notes Receivable [Member] | Financial and Insurance [Member]
Concentrations of Credit Risk
Loans	8,000
Additional unfunded commitments to extend credit	9,000
Commercial | Notes Receivable [Member] | Power and Utilities [Member]
Concentrations of Credit Risk
Loans	3,000
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
Concentrations of Credit Risk
Loans	16,895	$ 15,354
Commercial | Commercial mortgage | Notes Receivable [Member]
Concentrations of Credit Risk
Loans	18,000
Additional unfunded commitments to extend credit	$ 5,000