LOANS (Loans by credit quality indicator) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	$ 7,976,113	$ 7,410,881
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	7,415,084	6,947,494
Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	561,029	463,387
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	6,393,596	5,731,712
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	5,989,691	5,404,923
Commercial | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	403,905	326,789
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	2,276,916	2,020,231
Commercial | Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	2,125,500	1,880,579
Commercial | Commercial | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	151,416	139,652
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	4,116,680	3,711,481
Commercial | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	3,864,191	3,524,344
Commercial | Commercial real estate | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	252,489	187,137
Agricultural and agricultural real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	545,006	565,408
Agricultural and agricultural real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	426,383	471,642
Agricultural and agricultural real estate | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	118,623	93,766
Residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	589,793	673,603
Residential real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	563,603	645,478
Residential real estate | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	26,190	28,125
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	447,718	440,158
Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	435,407	425,451
Consumer | Nonpass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans receivable held to maturity	$ 12,311	$ 14,707