Investment securities - Fair Value and Unrealized Losses of Investment Securities in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	$ 151,070	$ 145,428
Less Than 12 Months, Unrealized Losses	2,066	2,470
12 Months or More, Fair Value	3,717	4,434
12 Months or More, Unrealized Losses	65	94
Total, Fair Value	154,787	149,862
Total, Unrealized Losses	2,131	2,564
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		1,035
Less Than 12 Months, Unrealized Losses		39
Total, Fair Value		1,035
Total, Unrealized Losses		39
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	51,467	55,166
Less Than 12 Months, Unrealized Losses	544	707
12 Months or More, Fair Value		226
Total, Fair Value	51,467	55,392
Total, Unrealized Losses	544	707
U.S. Treasury securities
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	17,411	7,438
Less Than 12 Months, Unrealized Losses	156	132
Total, Fair Value	17,411	7,438
Total, Unrealized Losses	156	132
U.S. Government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	62,591	59,460
Less Than 12 Months, Unrealized Losses	1,277	1,480
Total, Fair Value	62,591	59,460
Total, Unrealized Losses	1,277	1,480
Mortgage-backed Securities, U.S. Government agencies
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	5,769	5,917
Less Than 12 Months, Unrealized Losses	23	27
12 Months or More, Fair Value	1,337	1,496
12 Months or More, Unrealized Losses	13	20
Total, Fair Value	7,106	7,413
Total, Unrealized Losses	36	47
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	13,832	16,412
Less Than 12 Months, Unrealized Losses	66	85
12 Months or More, Fair Value	2,380	2,712
12 Months or More, Unrealized Losses	52	74
Total, Fair Value	16,212	19,124
Total, Unrealized Losses	$ 118	$ 159