CORE DEPOSIT PREMIUM AND MORTGAGE SERVICING RIGHTS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
OTHER INTANGIBLE ASSETS
Loans Serviced For Others				$ 1,730,000	$ 1,890,000
Mortgage banking income	$ 13,682	$ 6,685	$ 8,062
Changes in other intangible assets
Balance, beginning of period	14,718	15,596	17,849
Additions	3,314	1,582	1,612
Amortization	(6,167)	(2,460)	(3,865)
Balance, end of period	11,865	14,718	15,596
Gross carrying value, accumulated amortization and net carrying value related to intangible assets
Gross Carrying Value				115,551	112,237
Accumulated Amortization				(103,686)	(97,519)
Net Carrying Value	11,865	14,718	15,596	11,865	14,718
Estimated Amortization Expense
Net Carrying Value	11,865	14,718	15,596	11,865	14,718
Core Deposit Premium
Changes in other intangible assets
Balance, beginning of period	0	0	2,006
Additions	0	0	0
Amortization	0	0	(2,006)
Balance, end of period	0	0	0
Gross carrying value, accumulated amortization and net carrying value related to intangible assets
Gross Carrying Value				44,642	44,642
Accumulated Amortization				(44,642)	(44,642)
Net Carrying Value	0	0	0	0	0
Estimated Amortization Expense
Net Carrying Value	0	0	0	0	0
Mortgage Servicing Rights
OTHER INTANGIBLE ASSETS
Mortgage banking income	3,300	1,600	1,600
Changes in other intangible assets
Balance, beginning of period	14,718	15,596	15,843
Additions	3,314	1,582	1,612
Amortization	(6,167)	(2,460)	(1,859)
Balance, end of period	11,865	14,718	15,596
Gross carrying value, accumulated amortization and net carrying value related to intangible assets
Gross Carrying Value				70,909	67,595
Accumulated Amortization				(59,044)	(52,877)
Net Carrying Value	11,865	14,718	15,596	11,865	14,718
Estimated Amortization Expense
2019				4,725
2020				3,815
2021				3,145
2022				180
2023				0
Thereafter				0
Net Carrying Value	11,865	14,718	15,596	$ 11,865	$ 14,718
Other disclosures
Fair market value, beginning of period	15,820	17,696
Fair market value, end of period	$ 12,003	$ 15,820	$ 17,696
Measurement Input, Discount Rate [Member] | Mortgage Servicing Rights
OTHER INTANGIBLE ASSETS
Debt Instrument, Measurement Input				0.096	0.095
Measurement Input, Constant Prepayment Rate [Member] | Mortgage Servicing Rights
OTHER INTANGIBLE ASSETS
Debt Instrument, Measurement Input				0.176	0.142