LOANS (Loans receivable) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Loans and Leases [Line Items]
Loans receivable	$ 9,099,646		$ 8,367,917
Allowance for credit losses	(103,377)	$ (119,937)	(70,395)	$ (66,222)	$ (63,850)	$ (61,963)
Loans receivable, net	8,996,269		8,297,522
Accrued interest receivable	38,000
Commercial and industrial | Commercial and industrial
Loans and Leases [Line Items]
Loans receivable	2,303,646		2,530,809
Allowance for credit losses	(34,598)	(32,531)	(30,787)	(28,424)	(27,680)	(26,306)
Commercial and industrial | Paycheck Protection Program ("PPP")
Loans and Leases [Line Items]
Loans receivable	1,128,035		0
Allowance for credit losses	0	0	0
Commercial and industrial | Owner occupied commercial real estate
Loans and Leases [Line Items]
Loans receivable	1,494,902		1,472,704
Allowance for credit losses	(11,229)	(23,402)	(8,214)	(7,250)	(6,182)	(6,525)
Commercial and industrial | Non-owner occupied commercial real estate
Loans and Leases [Line Items]
Loans receivable	1,659,683		1,495,877
Allowance for credit losses	(12,127)	(10,162)	(7,802)	(7,747)	(7,482)	(7,430)
Commercial and industrial | Real estate construction
Loans and Leases [Line Items]
Loans receivable	917,765		1,027,081
Allowance for credit losses	(20,787)	(28,667)	(11,599)	(10,688)	(10,010)	(9,679)
Agricultural and agricultural real estate
Loans and Leases [Line Items]
Loans receivable	508,058		565,837
Allowance for credit losses	(5,341)	(5,701)	(5,673)	(5,370)	(5,986)	(4,914)
Residential real estate
Loans and Leases [Line Items]
Loans receivable	701,899		832,277
Allowance for credit losses	(9,671)	(9,304)	(1,504)	(1,805)	(1,592)	(1,813)
Consumer
Loans and Leases [Line Items]
Loans receivable	385,658		443,332
Allowance for credit losses	$ (9,624)	$ (10,170)	$ (4,816)	$ (4,938)	$ (4,918)	$ (5,296)