LOANS (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	$ 8,114		$ 7,554
Ending Balance Under ASC 450-20	58,108		54,409
Total	66,222	$ 63,850	61,963	$ 61,221	$ 61,324	$ 55,686
Ending Balance Evaluated for Impairment Under ASC 310-10-35	85,993		81,796
Ending Balance Evaluated for Impairment Under ASC 450-20	7,890,120		7,329,085
Total	7,976,113		7,410,881
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	6,393,596		5,731,712
Commercial | Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	6,124		5,733
Ending Balance Under ASC 450-20	19,161		18,772
Total	25,285	24,082	24,505	22,069	20,709	18,098
Ending Balance Evaluated for Impairment Under ASC 310-10-35	23,355		24,202
Ending Balance Evaluated for Impairment Under ASC 450-20	2,253,561		1,996,029
Total	2,276,916		2,020,231
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	308		218
Ending Balance Under ASC 450-20	29,073		25,320
Total	29,381	27,300	25,538	23,690	23,727	21,950
Ending Balance Evaluated for Impairment Under ASC 310-10-35	19,913		14,388
Ending Balance Evaluated for Impairment Under ASC 450-20	4,096,767		3,697,093
Total	4,116,680		3,711,481
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	1,084		686
Ending Balance Under ASC 450-20	4,267		4,267
Total	5,351	6,049	4,953	4,566	5,709	4,258
Ending Balance Evaluated for Impairment Under ASC 310-10-35	20,073		15,951
Ending Balance Evaluated for Impairment Under ASC 450-20	524,933		549,457
Total	545,006		565,408
Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	123		168
Ending Balance Under ASC 450-20	1,319		1,617
Total	1,442	1,572	1,785	1,850	1,857	2,224
Ending Balance Evaluated for Impairment Under ASC 310-10-35	17,552		20,251
Ending Balance Evaluated for Impairment Under ASC 450-20	572,241		653,352
Total	589,793		673,603
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	475		749
Ending Balance Under ASC 450-20	4,288		4,433
Total	4,763	$ 4,847	5,182	$ 9,046	$ 9,322	$ 9,156
Ending Balance Evaluated for Impairment Under ASC 310-10-35	5,100		7,004
Ending Balance Evaluated for Impairment Under ASC 450-20	442,618		433,154
Total	$ 447,718		$ 440,158