LOANS (Loans receivable) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Loans and Leases [Line Items]
Held to maturity	$ 10,050,456	$ 10,023,051
Allowance for credit losses	(130,172)	(131,606)	$ (97,350)	$ (70,395)
Loans receivable, net	9,920,284	9,891,445
Accrued interest receivable	41,600
Commercial and industrial | Commercial and industrial
Loans and Leases [Line Items]
Held to maturity	2,421,260	2,534,799
Allowance for credit losses	(36,095)	(38,818)	(32,455)	(34,207)
Commercial and industrial | Paycheck Protection Program ("PPP")
Loans and Leases [Line Items]
Held to maturity	1,155,328	957,785
Allowance for credit losses	0	0	0	0
Commercial and industrial | Owner occupied commercial real estate
Loans and Leases [Line Items]
Held to maturity	1,837,559	1,776,406
Allowance for credit losses	(19,416)	(20,001)	(10,337)	(7,921)
Commercial and industrial | Non-owner occupied commercial real estate
Loans and Leases [Line Items]
Held to maturity	1,967,183	1,921,481
Allowance for credit losses	(24,694)	(20,873)	(8,320)	(7,584)
Commercial and industrial | Real estate construction
Loans and Leases [Line Items]
Held to maturity	796,027	863,220
Allowance for credit losses	(19,931)	(20,080)	(22,951)	(8,677)
Agricultural and agricultural real estate
Loans and Leases [Line Items]
Held to maturity	683,969	714,526
Allowance for credit losses	(7,111)	(7,129)	(4,797)	(5,680)
Residential real estate
Loans and Leases [Line Items]
Held to maturity	786,994	840,442
Allowance for credit losses	(11,012)	(11,935)	(8,725)	(1,504)
Consumer
Loans and Leases [Line Items]
Held to maturity	402,136	414,392
Allowance for credit losses	$ (11,913)	$ (12,770)	$ (9,765)	$ (4,822)