Investment securities - Fair Value and Unrealized Losses of Investment Securities in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	$ 33,083	$ 98,260
Less Than 12 Months, Unrealized Losses	54	340
12 Months or More, Fair Value	4,785	13,991
12 Months or More, Unrealized Losses	75	216
Total, Fair Value	37,868	112,251
Total, Unrealized Losses	129	556
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	558
Less Than 12 Months, Unrealized Losses	1
12 Months or More, Fair Value		532
12 Months or More, Unrealized Losses		32
Total, Fair Value	558	532
Total, Unrealized Losses	1	32
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	3,244	21,341
Less Than 12 Months, Unrealized Losses	2	87
12 Months or More, Fair Value	632	1,952
12 Months or More, Unrealized Losses	1	19
Total, Fair Value	3,876	23,293
Total, Unrealized Losses	3	106
U.S. Treasury securities
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value		9,999
Less Than 12 Months, Unrealized Losses		31
Total, Fair Value		9,999
Total, Unrealized Losses		31
U.S. Government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	19,368	34,159
Less Than 12 Months, Unrealized Losses	40	62
Total, Fair Value	19,368	34,159
Total, Unrealized Losses	40	62
Mortgage-backed Securities, U.S. Government agencies
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	7,554	15,114
Less Than 12 Months, Unrealized Losses	8	56
12 Months or More, Fair Value	1,675	5,477
12 Months or More, Unrealized Losses	15	61
Total, Fair Value	9,229	20,591
Total, Unrealized Losses	23	117
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Less Than 12 Months, Fair Value	2,359	17,647
Less Than 12 Months, Unrealized Losses	3	104
12 Months or More, Fair Value	2,478	6,030
12 Months or More, Unrealized Losses	59	104
Total, Fair Value	4,837	23,677
Total, Unrealized Losses	$ 62	$ 208