Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	$ 244,940	$ 280,342
Available-for-sale, Gross Unrealized Gains	5,102	5,133
Available-for-sale, Gross Unrealized Losses	129	540
Available-for-sale, Fair Value	249,913	284,935
Held-to-maturity, Amortized Cost	10,864	12,109
Held-to-maturity, Gross Unrealized Gains	682	513
Held-to-maturity, Gross Unrealized Losses		16
Held-to-maturity, Fair Value	11,546	12,606
Mortgage-backed Securities, U.S. Government agencies
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	23,300	31,612
Available-for-sale, Gross Unrealized Gains	104	73
Available-for-sale, Gross Unrealized Losses	23	117
Available-for-sale, Fair Value	23,381	31,568
Held-to-maturity, Amortized Cost	72	84
Held-to-maturity, Gross Unrealized Gains	1	1
Held-to-maturity, Fair Value	73	85
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	24,731	32,928
Available-for-sale, Gross Unrealized Gains	156	119
Available-for-sale, Gross Unrealized Losses	62	208
Available-for-sale, Fair Value	24,825	32,839
Held-to-maturity, Amortized Cost	3,929	5,160
Held-to-maturity, Gross Unrealized Gains	257	326
Held-to-maturity, Fair Value	4,186	5,486
U.S. Government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	67,478	68,831
Available-for-sale, Gross Unrealized Gains	218	291
Available-for-sale, Gross Unrealized Losses	40	62
Available-for-sale, Fair Value	67,656	69,060
U.S. Treasury securities
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost		10,030
Available-for-sale, Gross Unrealized Losses		31
Available-for-sale, Fair Value		9,999
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	14,732	15,842
Available-for-sale, Gross Unrealized Gains	1,023	735
Available-for-sale, Gross Unrealized Losses	1	32
Available-for-sale, Fair Value	15,754	16,545
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Available-for-sale, Amortized Cost	114,699	121,099
Available-for-sale, Gross Unrealized Gains	3,601	3,915
Available-for-sale, Gross Unrealized Losses	3	90
Available-for-sale, Fair Value	118,297	124,924
Held-to-maturity, Amortized Cost	6,863	6,865
Held-to-maturity, Gross Unrealized Gains	424	186
Held-to-maturity, Gross Unrealized Losses		16
Held-to-maturity, Fair Value	$ 7,287	$ 7,035

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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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.

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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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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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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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