Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Details) $ in Thousands	Mar. 31, 2020 USD ($) company	Dec. 31, 2019 USD ($)
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	$ 293,687	$ 328,643
Available-for-sale, Gross Unrealized Gains	9,539	2,988
Available-for-sale, Gross Unrealized Losses	342	1,153
Available-for-sale, Fair Value	302,884	330,478
Held-to-maturity, Amortized Cost	7,520	7,656
Held-to-maturity, Gross Unrealized Gains	183	233
Held-to-maturity, Gross Unrealized Losses	16
Held-to-maturity, Fair value	7,687	7,889
Equity investments carried at fair value	$ 299	423
Number of equity securities portfolio consisting of stock of other financial institutions \| company	2
Fair value of equity portfolio in excess of cost basis	$ 22
Mortgage-backed Securities, U.S. government agencies
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	7,125	8,368
Available-for-sale, Gross Unrealized Gains	178	112
Available-for-sale, Gross Unrealized Losses	3	10
Available-for-sale, Fair Value	7,300	8,470
Held-to-maturity, Amortized Cost	28	31
Held-to-maturity, Fair value	28	31
Mortgage-backed Securities, U.S. government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	98,880	101,914
Available-for-sale, Gross Unrealized Gains	4,143	1,011
Available-for-sale, Gross Unrealized Losses	3	77
Available-for-sale, Fair Value	103,020	102,848
Held-to-maturity, Amortized Cost	641	773
Held-to-maturity, Gross Unrealized Gains	30	25
Held-to-maturity, Gross Unrealized Losses	1
Held-to-maturity, Fair value	670	798
Commercial mortgage-backed Securities, U.S. government-sponsored enterprises
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	12,675	12,694
Available-for-sale, Gross Unrealized Gains	638	171
Available-for-sale, Gross Unrealized Losses		4
Available-for-sale, Fair Value	13,313	12,861
U.S. government-sponsored enterprises state and municipals
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	83,028	87,156
Available-for-sale, Gross Unrealized Gains	1,562	181
Available-for-sale, Gross Unrealized Losses		227
Available-for-sale, Fair Value	84,590	87,110
U.S. Treasuries
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	21,471	23,966
Available-for-sale, Gross Unrealized Gains	650	162
Available-for-sale, Fair Value	22,121	24,128
Common equity securities.
Schedule of Trading Securities and Other Trading Assets
Equity investments carried at fair value	299	423
State and Municipals, Taxable
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	32,116	35,418
Available-for-sale, Gross Unrealized Gains	772	295
Available-for-sale, Gross Unrealized Losses	328	815
Available-for-sale, Fair Value	32,560	34,898
State and Municipals, Tax-exempt
Schedule of Trading Securities and Other Trading Assets
Available-for-sale, Amortized Cost	38,392	59,127
Available-for-sale, Gross Unrealized Gains	1,596	1,056
Available-for-sale, Gross Unrealized Losses	8	20
Available-for-sale, Fair Value	39,980	60,163
Held-to-maturity, Amortized Cost	6,851	6,852
Held-to-maturity, Gross Unrealized Gains	153	208
Held-to-maturity, Gross Unrealized Losses	15
Held-to-maturity, Fair value	$ 6,989	$ 7,060

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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