Fair value of financial instruments - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Interest receivable	$ 17,083	$ 14,503
Mortgage servicing rights		88,829	$ 76,107	$ 33,081
Derivatives	21,981	14,316
Financial liabilities:
Derivatives	17,933	11,637
Level 1
Financial assets:
Cash and cash equivalents	232,681	125,356
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	3,743,085	3,051,972
Deposits, With stated maturities	0	0
Securities sold under agreement to repurchase and federal funds sold	23,745	15,081
Federal Home Loan Bank advances	0	0
Subordinated debt	0	0
Interest Payable	376	530
Derivatives	0	0
Level 2
Financial assets:
Cash and cash equivalents	0	0
Investment securities	691,676	658,805
Loans, net	0	0
Loans held for sale	262,518	278,815
Interest receivable	3,282	2,848
Mortgage servicing rights	0	0
Derivatives	21,981	14,316
Financial liabilities:
Deposits, Without stated maturities	0	0
Deposits, With stated maturities	1,200,145	1,122,076
Securities sold under agreement to repurchase and federal funds sold	0	0
Federal Home Loan Bank advances	250,213	181,864
Subordinated debt	29,706	30,000
Interest Payable	6,089	4,485
Derivatives	17,933	11,637
Level 3
Financial assets:
Cash and cash equivalents	0	0
Investment securities	0	0
Loans, net	4,363,903	3,630,500
Loans held for sale	0	0
Interest receivable	13,801	11,655
Mortgage servicing rights	75,521	88,829
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
Interest Payable	0	0
Derivatives	0	0
Carrying amount
Financial assets:
Cash and cash equivalents	232,681	125,356
Investment securities	691,676	658,805
Loans, net	4,378,503	3,638,579
Loans held for sale	262,518	278,815
Interest receivable	17,083	14,503
Mortgage servicing rights	75,521	88,829
Derivatives	21,981	14,316
Financial liabilities:
Deposits, Without stated maturities	3,743,085	3,051,972
Deposits, With stated maturities	1,191,853	1,119,745
Securities sold under agreement to repurchase and federal funds sold	23,745	15,081
Federal Home Loan Bank advances	250,000	181,765
Subordinated debt	30,930	30,930
Interest Payable	6,465	5,015
Derivatives	17,933	11,637
Fair Value
Financial assets:
Cash and cash equivalents	232,681	125,356
Investment securities	691,676	658,805
Loans, net	4,363,903	3,630,500
Loans held for sale	262,518	278,815
Interest receivable	17,083	14,503
Mortgage servicing rights	75,521	88,829
Derivatives	21,981	14,316
Financial liabilities:
Deposits, Without stated maturities	3,743,085	3,051,972
Deposits, With stated maturities	1,200,145	1,122,076
Securities sold under agreement to repurchase and federal funds sold	23,745	15,081
Federal Home Loan Bank advances	250,213	181,864
Subordinated debt	29,706	30,000
Interest Payable	6,465	5,015
Derivatives	$ 17,933	$ 11,637

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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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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