Loans and Allowance for Loan Losses - Loans 90 Days or More Past Due and Still Accruing (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Loans disclosures
Loans 90 days or more past due and still accruing	$ 21	$ 20
Commercial
Loans disclosures
Loans 90 days or more past due and still accruing	6	3
Commercial | Commercial and industrial
Loans disclosures
Loans 90 days or more past due and still accruing	3	3
Commercial | Commercial mortgage
Loans disclosures
Loans 90 days or more past due and still accruing	3	0
Consumer
Loans disclosures
Loans 90 days or more past due and still accruing	15	17
Consumer | Residential mortgage and home equity
Loans disclosures
Loans 90 days or more past due and still accruing	0	0
Consumer | Other consumer
Loans disclosures
Loans 90 days or more past due and still accruing	$ 15	$ 17