Loans and Allowance for Loan Losses - Loans 90 Days or More Past Due and Still Accruing (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Loans disclosures
Loans 90 days or more past due and still accruing	$ 29	$ 45
Commercial
Loans disclosures
Loans 90 days or more past due and still accruing	20	24
Commercial | Commercial and industrial
Loans disclosures
Loans 90 days or more past due and still accruing	7	13
Commercial | Commercial mortgage
Loans disclosures
Loans 90 days or more past due and still accruing	13	11
Consumer
Loans disclosures
Loans 90 days or more past due and still accruing	9	21
Consumer | Residential mortgage and home equity
Loans disclosures
Loans 90 days or more past due and still accruing	0	7
Consumer | Other consumer
Loans disclosures
Loans 90 days or more past due and still accruing	$ 9	$ 14