Investment securities - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 3,250,000		$ 3,250,000		$ 3,107,000
Available-for-sale securities	656,095,000		656,095,000		$ 668,033,000
Net gains (losses) on marketable equity securities	$ 55,000	$ (27,000)	$ 151,000	$ (108,000)
Number of securities in securities portfolio | security	359		359		360
Number of securities in securities portfolio, unrealized loss position | security	57		57		174
Other than temporary impairment	$ 0	$ 0	$ 0	$ 0
Trade Date Payables
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale securities	3,671,000		3,671,000		$ 2,120,000
Collateral Pledged
Debt and Equity Securities, FV-NI [Line Items]
Securities pledged	$ 329,681,000		$ 329,681,000		$ 326,215,000