LOANS (Allowance for credit losses on financing receivables) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	$ 8,559	$ 7,554
Ending Balance Under ASC 450-20	54,080	54,409
Total	62,639	61,963	$ 58,656	$ 55,686
Ending Balance Evaluated for Impairment Under ASC 310-10-35	87,568	81,796
Ending Balance Evaluated for Impairment Under ASC 450-20	7,245,294	7,329,085
Total	7,332,862	7,410,881
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5,745,051	5,731,712
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	6,301	5,733
Ending Balance Under ASC 450-20	17,520	18,772
Total	23,821	24,505	19,395	18,098
Ending Balance Evaluated for Impairment Under ASC 310-10-35	24,498	24,202
Ending Balance Evaluated for Impairment Under ASC 450-20	2,018,096	1,996,029
Total	2,042,594	2,020,231
Commercial | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	353	218
Ending Balance Under ASC 450-20	26,434	25,320
Total	26,787	25,538	23,469	21,950
Ending Balance Evaluated for Impairment Under ASC 310-10-35	18,150	14,388
Ending Balance Evaluated for Impairment Under ASC 450-20	3,684,307	3,697,093
Total	3,702,457	3,711,481
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	1,099	686
Ending Balance Under ASC 450-20	4,499	4,267
Total	5,598	4,953	4,716	4,258
Ending Balance Evaluated for Impairment Under ASC 310-10-35	20,475	15,951
Ending Balance Evaluated for Impairment Under ASC 450-20	524,330	549,457
Total	544,805	565,408
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	153	168
Ending Balance Under ASC 450-20	1,452	1,617
Total	1,605	1,785	2,141	2,224
Ending Balance Evaluated for Impairment Under ASC 310-10-35	18,619	20,251
Ending Balance Evaluated for Impairment Under ASC 450-20	611,814	653,352
Total	630,433	673,603
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Ending Balance Under ASC 310-10-35	653	749
Ending Balance Under ASC 450-20	4,175	4,433
Total	4,828	5,182	$ 8,935	$ 9,156
Ending Balance Evaluated for Impairment Under ASC 310-10-35	5,826	7,004
Ending Balance Evaluated for Impairment Under ASC 450-20	406,747	433,154
Total	$ 412,573	$ 440,158