Investment securities - Fair Value and Unrealized Losses of Investment Securities in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	$ 2,589	$ 78,239
12 Months or More, Fair Value	443	43,281
Less Than 12 Months, Unrealized Losses	35	1,035
12 Months or More, Unrealized Losses	4	118
Total, Fair Value	3,032	121,520
Total, Unrealized Losses	39	1,153
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		23,929
Less Than 12 Months, Unrealized Losses		815
Total, Fair Value		23,929
Total, Unrealized Losses		815
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		2,684
12 Months or More, Fair Value		1,098
Less Than 12 Months, Unrealized Losses		19
12 Months or More, Unrealized Losses		1
Total, Fair Value		3,782
Total, Unrealized Losses		20
U.S. government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		13,695
12 Months or More, Fair Value		36,070
Less Than 12 Months, Unrealized Losses		149
12 Months or More, Unrealized Losses		78
Total, Fair Value		49,765
Total, Unrealized Losses		227
Mortgage-backed Securities, U.S. government agencies
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		992
12 Months or More, Fair Value	59	2,362
Less Than 12 Months, Unrealized Losses		1
12 Months or More, Unrealized Losses	1	9
Total, Fair Value	59	3,354
Total, Unrealized Losses	1	10
Commercial mortgage-backed Securities, U.S. government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	2,589
12 Months or More, Fair Value	384
Less Than 12 Months, Unrealized Losses	35
12 Months or More, Unrealized Losses	3
Total, Fair Value	2,973
Total, Unrealized Losses	$ 38
Mortgage-backed Securities, U.S. government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		36,939
12 Months or More, Fair Value		3,751
Less Than 12 Months, Unrealized Losses		51
12 Months or More, Unrealized Losses		30
Total, Fair Value		40,690
Total, Unrealized Losses		$ 81