FAIR VALUE MEASUREMENTS - Schedule of estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Financial assets
Debt securities held to maturity	$ 785,295
Portfolio loans, net	7,634,094	$ 7,101,111
Mortgage servicing rights	1,700,000
Financial liabilities
Short-term borrowings	351,054	17,678
Junior subordinated debt owed to unconsolidated trusts	71,810	71,635
Level 1 Inputs | Carrying Amount
Financial assets
Cash and cash equivalents	227,164	836,095
Level 1 Inputs | Fair Value
Financial assets
Cash and cash equivalents	227,164	836,095
Level 2 Inputs | Carrying Amount
Financial assets
Debt securities held to maturity	918,312	0
Loans held for sale	1,253	0
Accrued interest receivable	43,372	31,064
Financial liabilities
Time deposits	855,375	935,649
Securities sold under agreements to repurchase	229,806	270,139
Short-term borrowings	351,054	17,678
Long-term debt	30,000	46,056
Junior subordinated debt owed to unconsolidated trusts	71,810	71,635
Accrued interest payable	3,978	2,728
Level 2 Inputs | Fair Value
Financial assets
Debt securities held to maturity	785,295	0
Loans held for sale	1,276	0
Accrued interest receivable	43,372	31,064
Financial liabilities
Time deposits	830,596	935,778
Securities sold under agreements to repurchase	229,806	270,139
Short-term borrowings	351,085	17,673
Long-term debt	30,052	46,164
Junior subordinated debt owed to unconsolidated trusts	59,111	63,586
Accrued interest payable	3,978	2,728
Level 3 Inputs | Carrying Amount
Financial assets
Portfolio loans, net	7,634,094	7,101,111
Mortgage servicing rights	5,861	8,608
Other servicing rights	1,914	1,830
Level 3 Inputs | Fair Value
Financial assets
Portfolio loans, net	7,320,422	7,161,466
Mortgage servicing rights	18,284	12,133
Other servicing rights	2,331	2,268
Level 3 Inputs | Senior Notes | Carrying Amount
Financial liabilities
Long-term debt	0	39,944
Level 3 Inputs | Senior Notes | Fair Value
Financial liabilities
Long-term debt	0	40,400
Level 3 Inputs | Subordinated Debt | Carrying Amount
Financial liabilities
Long-term debt	222,038	182,773
Level 3 Inputs | Subordinated Debt | Fair Value
Financial liabilities
Long-term debt	$ 208,562	$ 195,600