Fair value of financial instruments - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial assets:
Interest receivable	$ 19,644	$ 17,083
Level 1
Financial assets:
Cash and cash equivalents	425,094	232,681
Investment securities	0	0
Loans, net	0	0
Loans held for sale	0	0
Interest receivable	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Financial liabilities:
Deposits, Without stated maturities	4,142,635	3,743,085
Deposits, With stated maturities	0	0
Securities sold under agreement to repurchase and federal funds sold	31,892	23,745
Federal Home Loan Bank advances	0	0
Subordinated debt	0	0
Other borrowings	0
Interest Payable	427	376
Derivatives	0	0
Level 2
Financial assets:
Cash and cash equivalents	0	0
Investment securities	767,575	691,676
Loans, net	0	0
Loans held for sale	325,304	262,518
Interest receivable	3,625	3,282
Mortgage servicing rights	0	0
Derivatives	66,281	21,981
Financial liabilities:
Deposits, Without stated maturities	0	0
Deposits, With stated maturities	1,243,610	1,200,145
Securities sold under agreement to repurchase and federal funds sold	0	0
Federal Home Loan Bank advances	259,037	250,213
Subordinated debt	28,921	29,706
Other borrowings	15,000
Interest Payable	8,466	6,089
Derivatives	66,696	17,933
Level 3
Financial assets:
Cash and cash equivalents	0	0
Investment securities	0	0
Loans, net	4,480,393	4,363,903
Loans held for sale	0	0
Interest receivable	16,019	13,801
Mortgage servicing rights	62,581	75,521
Derivatives	0	0
Financial liabilities:
Deposits, Without stated maturities	0	0
Deposits, With stated maturities	0	0
Securities sold under agreement to repurchase and federal funds sold	0	0
Federal Home Loan Bank advances	0	0
Subordinated debt	0	0
Other borrowings	0
Interest Payable	0	0
Derivatives	0	0
Carrying amount
Financial assets:
Cash and cash equivalents	425,094	232,681
Investment securities	767,575	691,676
Loans, net	4,478,897	4,378,503
Loans held for sale	325,304	262,518
Interest receivable	19,644	17,083
Mortgage servicing rights	62,581	75,521
Derivatives	66,281	21,981
Financial liabilities:
Deposits, Without stated maturities	4,142,635	3,743,085
Deposits, With stated maturities	1,234,297	1,191,853
Securities sold under agreement to repurchase and federal funds sold	31,892	23,745
Federal Home Loan Bank advances	250,000	250,000
Subordinated debt	30,930	30,930
Other borrowings	15,000
Interest Payable	8,893	6,465
Derivatives	66,696	17,933
Fair Value
Financial assets:
Cash and cash equivalents	425,094	232,681
Investment securities	767,575	691,676
Loans, net	4,480,393	4,363,903
Loans held for sale	325,304	262,518
Interest receivable	19,644	17,083
Mortgage servicing rights	62,581	75,521
Derivatives	66,281	21,981
Financial liabilities:
Deposits, Without stated maturities	4,142,635	3,743,085
Deposits, With stated maturities	1,243,610	1,200,145
Securities sold under agreement to repurchase and federal funds sold	31,892	23,745
Federal Home Loan Bank advances	259,037	250,213
Subordinated debt	28,921	29,706
Other borrowings	15,000
Interest Payable	8,893	6,465
Derivatives	$ 66,696	$ 17,933

Deposits with stated maturities fair value disclosure.

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Deposits without stated maturities fair value disclosure.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of interest payable on debt, including, but not limited to, trade payables.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity.

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Fair value portion of securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

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

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