LOANS (Past due financing receivables) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	$ 36,314	$ 16,650
Accruing Loans, Current	7,219,254	7,322,288
Nonaccrual	77,294	71,943
Total	7,332,862	7,410,881
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	25,361	15,363
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	9,247	561
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	1,706	726
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	23,513	8,324
Accruing Loans, Current	5,681,034	5,685,784
Nonaccrual	40,504	37,604
Total	5,745,051	5,731,712
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	14,123	7,393
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	7,801	205
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	1,589	726
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	11,437	2,779
Accruing Loans, Current	2,006,358	1,991,525
Nonaccrual	24,799	25,927
Total	2,042,594	2,020,231
Commercial | Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	8,253	2,574
Commercial | Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	2,859	205
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	325	0
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	12,076	5,545
Accruing Loans, Current	3,674,676	3,694,259
Nonaccrual	15,705	11,677
Total	3,702,457	3,711,481
Commercial | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	5,870	4,819
Commercial | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	4,942	0
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	1,264	726
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	3,010	99
Accruing Loans, Current	521,621	549,376
Nonaccrual	20,174	15,933
Total	544,805	565,408
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	2,179	99
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	800	0
Agricultural and agricultural real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	31	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	4,334	5,196
Accruing Loans, Current	613,667	655,329
Nonaccrual	12,432	13,078
Total	630,433	673,603
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	4,224	5,147
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	110	49
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	0	0
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	5,457	3,031
Accruing Loans, Current	402,932	431,799
Nonaccrual	4,184	5,328
Total	412,573	440,158
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	4,835	2,724
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	536	307
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Loans, Past Due	$ 86	$ 0