Investment securities - Narrative (Details)	3 Months Ended		12 Months Ended
	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Accrued interest receivable	$ 38,528,000	$ 43,603,000	$ 38,528,000	$ 43,603,000
Marketable securities at fair value	3,367,000	4,591,000	3,367,000	4,591,000
Trade date receivable - securities			0	0
Net gain on change in fair value and sale of equity securities			198,000	296,000	$ 148,000
Allowance for credit losses for investments	$ 0	$ 0	$ 0	$ 0
Number of securities in securities portfolio | security	511	514	511	514
Number of securities in securities portfolio, unrealized loss position | security	80	16	80	16
Provision for credit losses on available for sale debt securities	$ 0	$ 0
Collateral Pledged
Debt and Equity Securities, FV-NI [Line Items]
Securities pledged	1,226,646,000	804,821,000	$ 1,226,646,000	$ 804,821,000
Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Accrued interest receivable	$ 5,051,000	$ 4,540,000	$ 5,051,000	$ 4,540,000