Loans and asset quality - Loan Modifications (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modifications, CARES Act	$ 106	$ 282
Unpaid principal balance	174
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modifications, CARES Act	$ 56
Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding recorded investment post-modification			$ 4