LOANS (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	$ 8,362		$ 7,554
Ending Balance Under ASC 450-20	55,488		54,409
Total	63,850	$ 62,639	61,963	$ 61,324	$ 58,656	$ 55,686
Ending Balance Evaluated for Impairment Under ASC 310-10-35	94,718		81,796
Ending Balance Evaluated for Impairment Under ASC 450-20	7,760,567		7,329,085
Total	7,855,285		7,410,881
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	6,230,372		5,731,712
Commercial | Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	5,685		5,733
Ending Balance Under ASC 450-20	18,397		18,772
Total	24,082	23,821	24,505	20,709	19,395	18,098
Ending Balance Evaluated for Impairment Under ASC 310-10-35	26,881		24,202
Ending Balance Evaluated for Impairment Under ASC 450-20	2,211,572		1,996,029
Total	2,238,453		2,020,231
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	377		218
Ending Balance Under ASC 450-20	26,923		25,320
Total	27,300	26,787	25,538	23,727	23,469	21,950
Ending Balance Evaluated for Impairment Under ASC 310-10-35	22,141		14,388
Ending Balance Evaluated for Impairment Under ASC 450-20	3,969,778		3,697,093
Total	3,991,919		3,711,481
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	1,557		686
Ending Balance Under ASC 450-20	4,492		4,267
Total	6,049	5,598	4,953	5,709	4,716	4,258
Ending Balance Evaluated for Impairment Under ASC 310-10-35	21,729		15,951
Ending Balance Evaluated for Impairment Under ASC 450-20	527,675		549,457
Total	549,404		565,408
Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	197		168
Ending Balance Under ASC 450-20	1,375		1,617
Total	1,572	1,605	1,785	1,857	2,141	2,224
Ending Balance Evaluated for Impairment Under ASC 310-10-35	18,556		20,251
Ending Balance Evaluated for Impairment Under ASC 450-20	595,151		653,352
Total	613,707		673,603
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Ending Balance Under ASC 310-10-35	546		749
Ending Balance Under ASC 450-20	4,301		4,433
Total	4,847	$ 4,828	5,182	$ 9,322	$ 8,935	$ 9,156
Ending Balance Evaluated for Impairment Under ASC 310-10-35	5,411		7,004
Ending Balance Evaluated for Impairment Under ASC 450-20	456,391		433,154
Total	$ 461,802		$ 440,158