Fair value of financial instruments - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial assets:
Interest receivable	$ 14,503	$ 13,069
Mortgage servicing rights		76,107	$ 33,081	$ 29,711
Derivatives	14,316	9,690
Financial liabilities:
Derivatives	11,637	1,699
Level 1
Financial assets:
Cash and cash equivalents	125,356	119,751
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	3,051,972	2,976,066
Deposits, With stated maturities	0	0
Securities sold under agreement to repurchase and federal funds sold	15,081	14,293
Federal Home Loan Bank advances	0	190,000
Subordinated debt	0	0
Interest Payable	530	575
Derivatives	0	0
Level 2
Financial assets:
Cash and cash equivalents	0	0
Investment securities	658,805	540,388
Loans, net	0	3,064,373
Loans held for sale	278,815	526,185
Interest receivable	2,848	13,069
Mortgage servicing rights	0	0
Derivatives	14,316	9,690
Financial liabilities:
Deposits, Without stated maturities	0	0
Deposits, With stated maturities	1,122,076	682,403
Securities sold under agreement to repurchase and federal funds sold	0	0
Federal Home Loan Bank advances	181,864	112,465
Subordinated debt	30,000	36,670
Interest Payable	4,485	929
Derivatives	11,637	1,699
Level 3
Financial assets:
Cash and cash equivalents	0	0
Investment securities	0	3,604
Loans, net	3,630,500	77,027
Loans held for sale	0	0
Interest receivable	11,655	0
Mortgage servicing rights	88,829	76,107
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	125,356	119,751
Investment securities	658,805	543,992
Federal Home Loan Bank Stock		11,412
Loans, net	3,638,579	3,142,870
Loans held for sale	278,815	526,185
Interest receivable	14,503	13,069
Mortgage servicing rights	88,829	76,107
Derivatives	14,316	9,690
Financial liabilities:
Deposits, Without stated maturities	3,051,972	2,976,066
Deposits, With stated maturities	1,119,745	688,329
Securities sold under agreement to repurchase and federal funds sold	15,081	14,293
Federal Home Loan Bank advances	181,765	302,372
Subordinated debt	30,930	30,930
Interest Payable	5,015	1,504
Derivatives	11,637	1,699
Fair Value
Financial assets:
Cash and cash equivalents	125,356	119,751
Investment securities	658,805	543,992
Loans, net	3,630,500	3,141,400
Loans held for sale	278,815	526,185
Interest receivable	14,503	13,069
Mortgage servicing rights	88,829	76,107
Derivatives	14,316	9,690
Financial liabilities:
Deposits, Without stated maturities	3,051,972	2,976,066
Deposits, With stated maturities	1,122,076	682,403
Securities sold under agreement to repurchase and federal funds sold	15,081	14,293
Federal Home Loan Bank advances	181,864	302,465
Subordinated debt	30,000	36,670
Interest Payable	5,015	1,504
Derivatives	$ 11,637	$ 1,699

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 investments in Federal Home Loan Bank (FHLBank) stock.

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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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