FAIR VALUE (Estimated fair value financial instruments incl. carrying amounts) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Securities:
Carried at fair value	$ 2,400,460	$ 2,450,709
Held to maturity	95,240	245,341
Financial liabilities:
Deposits held for sale	118,564	106,409
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	492,501	273,630
Time deposits in other financial institutions	4,675	4,672
Securities:
Carried at fair value	20,458	25,414
Held to maturity	0	0
Other investments	0	0
Loans held for sale	0	0
Loans, net	0	0
Cash surrender value on life insurance	0	0
Derivative assets	0	0
Financial liabilities:
Short term borrowings	0	0
Other borrowings	0	0
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate lock commitments
Securities:
Derivative assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Demand deposits
Financial liabilities:
Deposits	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Savings deposits
Financial liabilities:
Deposits	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Time deposits
Financial liabilities:
Deposits	0	0
Deposits held for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Forward commitments
Financial liabilities:
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Time deposits in other financial institutions	0	0
Securities:
Carried at fair value	2,380,002	2,425,295
Held to maturity	95,240	245,341
Other investments	27,506	28,396
Loans held for sale	69,716	52,577
Loans, net	7,145,258	7,236,960
Cash surrender value on life insurance	163,764	162,892
Derivative assets	8,403	6,539
Financial liabilities:
Short term borrowings	104,314	227,010
Other borrowings	268,528	276,966
Derivative liabilities	10,858	6,044
Significant Other Observable Inputs (Level 2) | Interest rate lock commitments
Securities:
Derivative assets	0	0
Significant Other Observable Inputs (Level 2) | Demand deposits
Financial liabilities:
Deposits	3,118,909	3,264,737
Significant Other Observable Inputs (Level 2) | Savings deposits
Financial liabilities:
Deposits	5,145,929	5,107,962
Significant Other Observable Inputs (Level 2) | Time deposits
Financial liabilities:
Deposits	1,088,104	1,023,730
Deposits held for sale	111,742	0
Significant Other Observable Inputs (Level 2) | Forward commitments
Financial liabilities:
Derivative liabilities	282	399
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Time deposits in other financial institutions	0	0
Securities:
Carried at fair value	0	0
Held to maturity	0	0
Other investments	0	0
Loans held for sale	0	67,224
Loans, net	21,487	25,509
Cash surrender value on life insurance	0	0
Derivative assets	0	0
Financial liabilities:
Short term borrowings	0	0
Other borrowings	0	0
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Interest rate lock commitments
Securities:
Derivative assets	800	725
Significant Unobservable Inputs (Level 3) | Demand deposits
Financial liabilities:
Deposits	0	0
Significant Unobservable Inputs (Level 3) | Savings deposits
Financial liabilities:
Deposits	0	0
Significant Unobservable Inputs (Level 3) | Time deposits
Financial liabilities:
Deposits	0	0
Deposits held for sale	0	100,241
Significant Unobservable Inputs (Level 3) | Forward commitments
Financial liabilities:
Derivative liabilities	0	0
Commercial | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial
Securities:
Loans, net	0	0
Commercial | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial real estate
Securities:
Loans, net	0	0
Commercial | Significant Other Observable Inputs (Level 2) | Commercial
Securities:
Loans, net	1,962,582	1,942,675
Commercial | Significant Other Observable Inputs (Level 2) | Commercial real estate
Securities:
Loans, net	3,643,425	3,666,733
Commercial | Significant Unobservable Inputs (Level 3) | Commercial
Securities:
Loans, net	9,998	12,932
Commercial | Significant Unobservable Inputs (Level 3) | Commercial real estate
Securities:
Loans, net	717	405
Agricultural and agricultural real estate | Quoted Prices in Active Markets for Identical Assets (Level 1)
Securities:
Loans, net	0	0
Agricultural and agricultural real estate | Significant Other Observable Inputs (Level 2)
Securities:
Loans, net	522,885	542,042
Agricultural and agricultural real estate | Significant Unobservable Inputs (Level 3)
Securities:
Loans, net	8,932	11,070
Residential real estate | Quoted Prices in Active Markets for Identical Assets (Level 1)
Securities:
Loans, net	0	0
Residential real estate | Significant Other Observable Inputs (Level 2)
Securities:
Loans, net	611,879	654,118
Residential real estate | Significant Unobservable Inputs (Level 3)
Securities:
Loans, net	1,207	478
Consumer | Quoted Prices in Active Markets for Identical Assets (Level 1)
Securities:
Loans, net	0	0
Consumer | Significant Other Observable Inputs (Level 2)
Securities:
Loans, net	404,487	431,392
Consumer | Significant Unobservable Inputs (Level 3)
Securities:
Loans, net	633	624
Carrying Amount
Financial assets:
Cash and cash equivalents	492,501	273,630
Time deposits in other financial institutions	4,675	4,672
Securities:
Carried at fair value	2,400,460	2,450,709
Held to maturity	88,089	236,283
Other investments	27,506	28,396
Loans held for sale	69,716	119,801
Loans, net	7,268,905	7,345,734
Cash surrender value on life insurance	163,764	162,892
Derivative assets	8,403	6,539
Financial liabilities:
Short term borrowings	104,314	227,010
Other borrowings	268,312	274,905
Derivative liabilities	10,858	6,044
Carrying Amount | Interest rate lock commitments
Securities:
Derivative assets	800	725
Carrying Amount | Demand deposits
Financial liabilities:
Deposits	3,118,909	3,264,737
Carrying Amount | Savings deposits
Financial liabilities:
Deposits	5,145,929	5,107,962
Carrying Amount | Time deposits
Financial liabilities:
Deposits	1,008,104	1,023,730
Deposits held for sale	118,564	106,409
Carrying Amount | Forward commitments
Financial liabilities:
Derivative liabilities	282	399
Carrying Amount | Commercial | Commercial
Securities:
Loans, net	2,018,438	1,994,785
Carrying Amount | Commercial | Commercial real estate
Securities:
Loans, net	3,675,063	3,684,213
Carrying Amount | Agricultural and agricultural real estate
Securities:
Loans, net	540,022	561,265
Carrying Amount | Residential real estate
Securities:
Loans, net	627,619	670,473
Carrying Amount | Consumer
Securities:
Loans, net	407,763	434,998
Estimated Fair Value
Financial assets:
Cash and cash equivalents	492,501	273,630
Time deposits in other financial institutions	4,675	4,672
Securities:
Carried at fair value	2,400,460	2,450,709
Held to maturity	95,240	245,341
Other investments	27,506	28,396
Loans held for sale	69,716	119,801
Loans, net	7,166,745	7,262,469
Cash surrender value on life insurance	163,764	162,892
Derivative assets	8,403	6,539
Financial liabilities:
Short term borrowings	104,314	227,010
Other borrowings	268,528	276,966
Derivative liabilities	10,858	6,044
Estimated Fair Value | Interest rate lock commitments
Securities:
Derivative assets	800	725
Estimated Fair Value | Demand deposits
Financial liabilities:
Deposits	3,118,909	3,264,737
Estimated Fair Value | Savings deposits
Financial liabilities:
Deposits	5,145,929	5,107,962
Estimated Fair Value | Time deposits
Financial liabilities:
Deposits	1,088,104	1,023,730
Deposits held for sale	111,742	100,241
Estimated Fair Value | Forward commitments
Financial liabilities:
Derivative liabilities	282	399
Estimated Fair Value | Commercial | Commercial
Securities:
Loans, net	1,972,580	1,955,607
Estimated Fair Value | Commercial | Commercial real estate
Securities:
Loans, net	3,644,142	3,667,138
Estimated Fair Value | Agricultural and agricultural real estate
Securities:
Loans, net	531,817	553,112
Estimated Fair Value | Residential real estate
Securities:
Loans, net	613,086	654,596
Estimated Fair Value | Consumer
Securities:
Loans, net	$ 405,120	$ 432,016