Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	$ 263,818	$ 258,857
Available-for-sale, Gross Unrealized Gains	2,874	3,050
Available-for-sale, Gross Unrealized Losses	2,048	2,497
Available-for-sale, Fair Value	264,644	259,410
Held-to-maturity, Amortized Cost	10,180	10,517
Held-to-maturity, Gross Unrealized Gains	262	264
Held-to-maturity, Gross Unrealized Losses	83	67
Held-to-maturity, Fair value	10,359	10,714
Mortgage-backed Securities, U.S. Government agencies
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	19,321	21,041
Available-for-sale, Gross Unrealized Gains	47	48
Available-for-sale, Gross Unrealized Losses	36	47
Available-for-sale, Fair Value	19,332	21,042
Held-to-maturity, Amortized Cost	64	68
Held-to-maturity, Gross Unrealized Gains		1
Held-to-maturity, Fair value	64	69
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	20,213	22,303
Available-for-sale, Gross Unrealized Gains	46	48
Available-for-sale, Gross Unrealized Losses	118	159
Available-for-sale, Fair Value	20,141	22,192
Held-to-maturity, Amortized Cost	3,255	3,587
Held-to-maturity, Gross Unrealized Gains	176	191
Held-to-maturity, Fair value	3,431	3,778
U.S. Government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	83,677	82,314
Available-for-sale, Gross Unrealized Gains	72	79
Available-for-sale, Gross Unrealized Losses	1,277	1,480
Available-for-sale, Fair Value	82,472	80,913
U.S. Treasury securities
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	20,050	7,570
Available-for-sale, Gross Unrealized Gains	1
Available-for-sale, Gross Unrealized Losses	156	132
Available-for-sale, Fair Value	19,895	7,438
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	14,663	14,698
Available-for-sale, Gross Unrealized Gains	604	566
Available-for-sale, Gross Unrealized Losses		39
Available-for-sale, Fair Value	15,267	15,225
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	105,894	110,931
Available-for-sale, Gross Unrealized Gains	2,104	2,309
Available-for-sale, Gross Unrealized Losses	461	640
Available-for-sale, Fair Value	107,537	112,600
Held-to-maturity, Amortized Cost	6,861	6,862
Held-to-maturity, Gross Unrealized Gains	86	72
Held-to-maturity, Gross Unrealized Losses	83	67
Held-to-maturity, Fair value	$ 6,864	$ 6,867

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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