Fair value of financial instruments - Estimated Fair Values and Carrying Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financial assets:
Loans held for sale	$ 1,009,762		$ 899,173
Interest receivable	44,393	$ 43,603
Carrying amount
Financial assets:
Cash and cash equivalents	1,895,133	1,317,898
Investment securities	1,229,845	1,176,991
Loans, net	6,889,388	6,912,570
Loans held for sale	1,009,762	899,173
Interest receivable	44,393	43,603
Mortgage servicing rights	104,192	79,997
Derivatives	65,203	68,938
Financial liabilities:
Deposits, Without stated maturities	8,876,456	8,020,783
Deposits, With stated maturities	1,380,430	1,437,254
Securities sold under agreement to repurchase and federal funds sold	29,234	32,199
Subordinated debt	149,369	189,527
Other borrowings	1,576	16,598
Interest payable	4,187	6,772
Derivatives	30,752	48,242
Aggregate fair value
Financial assets:
Cash and cash equivalents	1,895,133	1,317,898
Investment securities	1,229,845	1,176,991
Loans, net	7,042,144	7,058,693
Loans held for sale	1,009,762	899,173
Interest receivable	44,393	43,603
Mortgage servicing rights	104,192	79,997
Derivatives	65,203	68,938
Financial liabilities:
Deposits, Without stated maturities	8,876,456	8,020,783
Deposits, With stated maturities	1,391,861	1,446,605
Securities sold under agreement to repurchase and federal funds sold	29,234	32,199
Subordinated debt	150,954	192,149
Other borrowings	1,576	16,598
Interest payable	4,187	6,772
Derivatives	30,752	48,242
Aggregate fair value \| Level 1
Financial assets:
Cash and cash equivalents	1,895,133	1,317,898
Investment securities	0	0
Loans, net	0	0
Loans held for sale	0	0
Interest receivable	30	33
Mortgage servicing rights	0	0
Derivatives	0	0
Financial liabilities:
Deposits, Without stated maturities	8,876,456	8,020,783
Deposits, With stated maturities	0	0
Securities sold under agreement to repurchase and federal funds sold	29,234	32,199
Subordinated debt	0	0
Other borrowings	0	0
Interest payable	246	327
Derivatives	0	0
Aggregate fair value \| Level 2
Financial assets:
Cash and cash equivalents	0	0
Investment securities	1,229,845	1,176,991
Loans, net	0	0
Loans held for sale	834,779	683,770
Interest receivable	5,376	5,254
Mortgage servicing rights	0	0
Derivatives	65,203	68,938
Financial liabilities:
Deposits, Without stated maturities	0	0
Deposits, With stated maturities	1,391,861	1,446,605
Securities sold under agreement to repurchase and federal funds sold	0	0
Subordinated debt	0	0
Other borrowings	1,576	16,598
Interest payable	3,110	4,210
Derivatives	30,752	48,242
Aggregate fair value \| Level 3
Financial assets:
Cash and cash equivalents	0	0
Investment securities	0	0
Loans, net	7,042,144	7,058,693
Loans held for sale	174,983	215,403
Interest receivable	38,987	38,316
Mortgage servicing rights	104,192	79,997
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
Subordinated debt	150,954	192,149
Other borrowings	0	0
Interest payable	831	2,235
Derivatives	$ 0	$ 0

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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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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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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