LOANS (Loans by credit quality indicator) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 7,332,862	$ 7,410,881
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	6,861,050	6,947,494
Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	471,812	463,387
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	5,745,051	5,731,712
Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	5,417,781	5,404,923
Commercial | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	327,270	326,789
Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	2,042,594	2,020,231
Commercial | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	1,907,620	1,880,579
Commercial | Commercial | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	134,974	139,652
Commercial | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,702,457	3,711,481
Commercial | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	3,510,161	3,524,344
Commercial | Commercial real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	192,296	187,137
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	544,805	565,408
Agricultural and agricultural real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	440,927	471,642
Agricultural and agricultural real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	103,878	93,766
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	630,433	673,603
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	603,647	645,478
Residential real estate | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	26,786	28,125
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	412,573	440,158
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	398,695	425,451
Consumer | Nonpass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans receivable held to maturity	$ 13,878	$ 14,707