Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Details) $ in Thousands	Mar. 31, 2019 USD ($) company	Dec. 31, 2018 USD ($)
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	$ 271,196	$ 272,933
Available-for-sale, Gross Unrealized Gains	1,331	757
Available-for-sale, Gross Unrealized Losses	2,143	4,008
Available-for-sale, Fair Value	270,384	269,682
Held-to-maturity, Amortized Cost	8,162	8,361
Held-to-maturity, Gross Unrealized Gains	151	67
Held-to-maturity, Gross Unrealized Losses	3	48
Held-to-maturity, Fair value	8,310	8,380
Equity investments carried at fair value	$ 292	291
Number of equity securities portfolio consisting of stock of other financial institutions \| company	2
Fair value of equity portfolio in excess of cost basis	$ 15
Mortgage-backed Securities, U.S. Government agencies
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	11,716	12,663
Available-for-sale, Gross Unrealized Gains	80	50
Available-for-sale, Gross Unrealized Losses	56	84
Available-for-sale, Fair Value	11,740	12,629
Held-to-maturity, Amortized Cost	40	42
Held-to-maturity, Fair value	40	42
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	39,064	33,149
Available-for-sale, Gross Unrealized Gains	408	49
Available-for-sale, Gross Unrealized Losses	202	401
Available-for-sale, Fair Value	39,270	32,797
Held-to-maturity, Amortized Cost	1,267	1,464
Held-to-maturity, Gross Unrealized Gains	32	55
Held-to-maturity, Gross Unrealized Losses	3	5
Held-to-maturity, Fair value	1,296	1,514
Commercial mortgage-backed Securities, U.S. Government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	6,258	6,269
Available-for-sale, Gross Unrealized Losses	111	230
Available-for-sale, Fair Value	6,147	6,039
U.S. Government-sponsored enterprises state and municipals
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	93,914	94,999
Available-for-sale, Gross Unrealized Gains	9	2
Available-for-sale, Gross Unrealized Losses	1,373	2,183
Available-for-sale, Fair Value	92,550	92,818
U.S. Treasuries
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	23,954	25,948
Available-for-sale, Gross Unrealized Gains	35	9
Available-for-sale, Gross Unrealized Losses	232	365
Available-for-sale, Fair Value	23,757	25,592
Common equity securities.
Schedule of Trading Securities and Other Trading Assets
Equity investments carried at fair value	292	291
State and Municipals, Taxable
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	13,017	13,544
Available-for-sale, Gross Unrealized Gains	328	309
Available-for-sale, Fair Value	13,345	13,853
State and Municipals, Tax-exempt
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	83,273	86,361
Available-for-sale, Gross Unrealized Gains	471	338
Available-for-sale, Gross Unrealized Losses	169	745
Available-for-sale, Fair Value	83,575	85,954
Held-to-maturity, Amortized Cost	6,855	6,855
Held-to-maturity, Gross Unrealized Gains	119	12
Held-to-maturity, Gross Unrealized Losses		43
Held-to-maturity, Fair value	$ 6,974	$ 6,824

The amount the fair value of the equity portfolio exceeded the cost basis.

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

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost 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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