Loans and Allowance for Loan Losses - Loans 90 Days or More Past Due and Still Accruing (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Loans disclosures
Loans 90 days or more past due and still accruing	$ 18	$ 20
Commercial
Loans disclosures
Loans 90 days or more past due and still accruing	2	3
Commercial | Commercial and industrial
Loans disclosures
Loans 90 days or more past due and still accruing	2	3
Consumer
Loans disclosures
Loans 90 days or more past due and still accruing	16	17
Consumer | Other Consumer Loans [Member]
Loans disclosures
Loans 90 days or more past due and still accruing	$ 16	$ 17