Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) security
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	$ 273,742	$ 272,933
Available-for-sale, Gross Unrealized Gains	1,666	757
Available-for-sale, Gross Unrealized Losses	2,906	4,008
Available-for-sale, Fair Value	272,502	269,682
Available-for-sale, Fair Value	272,502	269,682
Held-to-maturity, Amortized Cost	9,274	8,361
Held-to-maturity, Gross Unrealized Gains	290	67
Held-to-maturity, Gross Unrealized Losses	17	48
Held-to-maturity, Fair value	9,547	8,380
Equity investments carried at fair value	46	$ 291
Number of equity securities portfolio consisting of stock of other financial institutions \| security		2
Net unrealized gains recognized in AOCI	3
State and Municipals Tax-exempt Bonds
Schedule of Trading Securities and Other Trading Assets
Held-to-maturity, Amortized Cost		$ 6,855
Held-to-maturity, Gross Unrealized Gains		12
Held-to-maturity, Gross Unrealized Losses		43
Held-to-maturity, Fair value		6,824
Mortgage-backed Securities, U.S. Government agencies
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	14,517	12,663
Available-for-sale, Gross Unrealized Gains	2	50
Available-for-sale, Gross Unrealized Losses	85	84
Available-for-sale, Fair Value	14,434	12,629
Held-to-maturity, Amortized Cost	54	42
Held-to-maturity, Fair value	54	42
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	19,752	33,149
Available-for-sale, Gross Unrealized Gains	10	49
Available-for-sale, Gross Unrealized Losses	231	401
Available-for-sale, Fair Value	19,531	32,797
Held-to-maturity, Amortized Cost	2,361	1,464
Held-to-maturity, Gross Unrealized Gains	138	55
Held-to-maturity, Gross Unrealized Losses	4	5
Held-to-maturity, Fair value	2,495	1,514
Commercial mortgage-backed Securities, U.S. Government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	6,315	6,269
Available-for-sale, Gross Unrealized Losses	120	230
Available-for-sale, Fair Value	6,195	6,039
U.S. Government-sponsored enterprises state and municipals
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	95,358	94,999
Available-for-sale, Gross Unrealized Gains	30	2
Available-for-sale, Gross Unrealized Losses	1,740	2,183
Available-for-sale, Fair Value	93,648	92,818
U.S. Treasuries
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	20,042	25,948
Available-for-sale, Gross Unrealized Gains		9
Available-for-sale, Gross Unrealized Losses	228	365
Available-for-sale, Fair Value	19,814	25,592
State and Municipals, Taxable
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	14,559	13,544
Available-for-sale, Gross Unrealized Gains	488	309
Available-for-sale, Fair Value	15,047	13,853
State and Municipals, Tax-exempt
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	103,199	86,361
Available-for-sale, Gross Unrealized Gains	1,136	338
Available-for-sale, Gross Unrealized Losses	502	745
Available-for-sale, Fair Value	103,833	$ 85,954
Held-to-maturity, Amortized Cost	6,859
Held-to-maturity, Gross Unrealized Gains	152
Held-to-maturity, Gross Unrealized Losses	13
Held-to-maturity, Fair value	$ 6,998

The number of equity securities portfolio consisting of stock of other financial institutions.

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

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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 of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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