FAIR VALUE - Quantitative Information about Level 3 Value Measurements - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Agricultural | Sales comparison approach | Average
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	30.00%
Impaired Loans | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	0.00%
Impaired Loans | Maximum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	50.00%
Impaired Loans | Sales comparison approach | Average
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	0.00%
Impaired Loans | Commercial and industrial | Sales comparison approach | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	0.00%
Impaired Loans | Commercial and industrial | Sales comparison approach | Maximum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		30.00%
Impaired Loans | Farm | Sales comparison approach
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		10.00%
Impaired Loans | Farm | Sales comparison approach | Average
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	30.00%	18.00%
Impaired Loans | Other real estate | Sales comparison approach
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value		$ 560
Impaired Loans | Other real estate | Sales comparison approach | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		16.00%
Impaired Loans | Other real estate | Sales comparison approach | Maximum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		31.00%
Impaired Loans | Other real estate | Sales comparison approach | Average
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	22.00%	27.00%
Impaired Loans | Other | Sales comparison approach | Average
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		10.00%
Other real estate owned | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	0.00%
Other real estate owned | Maximum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	50.00%
Mortgage servicing rights | Cash flow analysis
Unobservable Inputs
Discount rate (as a percent)	13.00%	14.00%
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		10.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 2,909	$ 2,537
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Commercial and industrial | Sales comparison approach
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 270	260
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	0.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Agricultural | Sales comparison approach
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 613	972
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	30.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Farm | Sales comparison approach
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 275	$ 745
Unobservable Inputs
Discount rate (as a percent)	30.00%	10.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Other real estate | Sales comparison approach
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 1,751
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Other real estate | Sales comparison approach | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	20.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Other real estate | Sales comparison approach | Maximum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)	30.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Impaired Loans | Other | Sales comparison approach | Minimum
Unobservable Inputs
Adjustment for differences between comparable sales, type of property, current status of property (as a percent)		10.00%
Non-recurring basis | Significant Unobservable Inputs (Level 3) | Mortgage servicing rights | Cash flow analysis
Quantitative information about level 3 fair value measurements for financial instruments measured at fair value
Fair Value	$ 400	$ 1,083
Unobservable Inputs
Discount rate (as a percent)	13.00%	14.00%