Loans and asset quality - Loan Modifications (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2019	Mar. 31, 2019
Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding recorded investment post-modification (less than)	$ 1	$ 1

Amount, after troubled debt restructuring, of modified financing receivable.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration