LOANS (Past due financing receivables) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 22,518	$ 16,650
Current	7,881,387	7,322,288
Nonaccrual	72,208	71,943
Total	7,976,113	7,410,881
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	17,099	15,363
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,379	561
90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	40	726
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	15,218	8,324
Current	6,341,676	5,685,784
Nonaccrual	36,702	37,604
Total	6,393,596	5,731,712
Commercial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	10,551	7,393
Commercial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,633	205
Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	34	726
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	9,137	2,779
Current	2,244,365	1,991,525
Nonaccrual	23,414	25,927
Total	2,276,916	2,020,231
Commercial | Commercial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,727	2,574
Commercial | Commercial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,376	205
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	34	0
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	6,081	5,545
Current	4,097,311	3,694,259
Nonaccrual	13,288	11,677
Total	4,116,680	3,711,481
Commercial | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,824	4,819
Commercial | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,257	0
Commercial | Commercial real estate | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	726
Agricultural and agricultural real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	3,168	99
Current	521,910	549,376
Nonaccrual	19,928	15,933
Total	545,006	565,408
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,144	99
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	24	0
Agricultural and agricultural real estate | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Residential real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	2,178	5,196
Current	576,365	655,329
Nonaccrual	11,250	13,078
Total	589,793	673,603
Residential real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,993	5,147
Residential real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	185	49
Residential real estate | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	1,954	3,031
Current	441,436	431,799
Nonaccrual	4,328	5,328
Total	447,718	440,158
Consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,411	2,724
Consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	537	307
Consumer | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 6	$ 0