Investment securities - Narrative (Details)	3 Months Ended
Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	$ 19,644,000		$ 17,083,000
Equity securities, at fair value	3,358,000		3,295,000
Available-for-sale securities	736,172,000		$ 676,705,000
Net gains (losses) on marketable equity securities	$ 63,000	$ 49,000
Number of securities in securities portfolio \| security	440		365
Number of securities in securities portfolio, unrealized loss position \| security	62		58
Allowance for credit losses for investments	$ 0
Provision for Credit Losses on Available for Sale Debt Securities	0
Other than temporary impairment		$ 0
Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Interest receivable	3,218,000		$ 2,843,000
Trade Date Payables
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale securities	8,273,000		0
Collateral Pledged
Debt and Equity Securities, FV-NI [Line Items]
Securities pledged	$ 411,276,000		$ 373,674,000

Number of available for sale securities in portfolio.

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Provision for Credit Losses on Available for Sale Debt Securities

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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