Investment securities - Fair Value and Unrealized Losses of Investment Securities in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	$ 288,190	$ 27,585
12 Months or More, Fair Value	11,352	284
Less Than 12 Months, Unrealized Losses	5,204	202
12 Months or More, Unrealized Losses	335	1
Total, Fair Value	299,542	27,869
Total, Unrealized Losses	5,539	203
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	29,122	9,246
12 Months or More, Fair Value	4,283
Less Than 12 Months, Unrealized Losses	771	30
12 Months or More, Unrealized Losses	178
Total, Fair Value	33,405	9,246
Total, Unrealized Losses	949	30
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	52,825	6,786
Less Than 12 Months, Unrealized Losses	1,087	37
Total, Fair Value	52,825	6,786
Total, Unrealized Losses	1,087	37
U.S. Treasuries
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	102,789
Less Than 12 Months, Unrealized Losses	610
Total, Fair Value	102,789
Total, Unrealized Losses	610
Mortgage-backed Securities, U.S. government agencies
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	11,279
Less Than 12 Months, Unrealized Losses	195
Total, Fair Value	11,279
Total, Unrealized Losses	195
Mortgage-backed Securities, U.S. government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	89,214	11,553
12 Months or More, Fair Value	7,069	284
Less Than 12 Months, Unrealized Losses	2,502	135
12 Months or More, Unrealized Losses	157	1
Total, Fair Value	96,283	11,837
Total, Unrealized Losses	2,659	$ 136
Corporate debt securities
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	2,961
Less Than 12 Months, Unrealized Losses	39
Total, Fair Value	2,961
Total, Unrealized Losses	$ 39

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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