FAIR VALUE - Fair Value of Assets and Liabilities Measured on a Nonrecurring Basis - (Details) - Non-recurring basis - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets:
Impaired servicing rights	$ 400	$ 1,083
Total impaired loans	2,909	2,537
Recorded investment of impaired loans	6,169	4,008
Valuation allowance of impaired loans	3,260	1,471
Additional provision for loan losses	2,214	1,158
Gross outstanding balance of servicing rights	525	1,283
Valuation allowance of servicing rights	125	200
Servicing right recovery included in earnings	75	0
Commercial and industrial
Assets:
Total impaired loans	270	260
Recorded investment of impaired loans	1,902
Valuation allowance of impaired loans	1,632
Agricultural
Assets:
Total impaired loans	613	972
Recorded investment of impaired loans	1,438
Valuation allowance of impaired loans	825
Farm
Assets:
Total impaired loans	275	745
Recorded investment of impaired loans	708
Valuation allowance of impaired loans	433
Other real estate
Assets:
Total impaired loans	1,751	560
Recorded investment of impaired loans	2,121
Valuation allowance of impaired loans	370
Significant Unobservable Inputs (Level 3)
Assets:
Impaired servicing rights	400	1,083
Total impaired loans	2,909	2,537
Significant Unobservable Inputs (Level 3) | Commercial and industrial
Assets:
Total impaired loans	270	260
Significant Unobservable Inputs (Level 3) | Agricultural
Assets:
Total impaired loans	613	972
Significant Unobservable Inputs (Level 3) | Farm
Assets:
Total impaired loans	275	745
Significant Unobservable Inputs (Level 3) | Other real estate
Assets:
Total impaired loans	$ 1,751	$ 560