Investment securities - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	$ 47,120,000		$ 47,120,000		$ 17,083,000
Equity securities, at fair value	4,389,000		4,389,000		3,295,000
Trade date payable - securities			1,214,000	$ 3,671,000	$ 0
Net (losses) gains on marketable equity securities	$ 59,000	$ 55,000	$ 94,000	151,000
Number of securities in securities portfolio | security	553		553		365
Number of securities in securities portfolio, unrealized loss position | security	33		33		58
Allowance for credit losses for investments	$ 0		$ 0
Provision for credit losses on available for sale debt securities	0		0
Other than temporary impairment		$ 0		$ 0
Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	4,753,000		4,753,000		$ 2,843,000
Collateral Pledged
Debt and Equity Securities, FV-NI [Line Items]
Securities pledged	$ 607,304,000		$ 607,304,000		$ 373,674,000