Investment securities - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	$ 26,587,000		$ 26,587,000		$ 17,083,000
Equity securities, at fair value	4,329,000		4,329,000		3,295,000
Trade date payable - securities			5,431,000	$ 1,089,000	$ 0
Net (losses) gains on marketable equity securities	$ (28,000)	$ 47,000	$ 35,000	96,000
Number of securities in securities portfolio | security	445		445		365
Number of securities in securities portfolio, unrealized loss position | security	8		8		58
Allowance for credit losses for investments	$ 0		$ 0
Provision for credit losses on available for sale debt securities			0
Other than temporary impairment		$ 0		$ 0
Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	3,184,000		3,184,000		$ 2,843,000
Collateral Pledged
Debt and Equity Securities, FV-NI [Line Items]
Securities pledged	$ 475,645,000		$ 475,645,000		$ 373,674,000