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Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($)
6 Months Ended 12 Months Ended
Jan. 01, 2019
Jun. 30, 2019
Dec. 31, 2018
Goodwill, Impairment Loss     $ 0
Goodwill, Period Increase (Decrease), Total   $ 0  
Accounting Standards Update 2018-02 [Member]      
Tax Cuts and Jobs Act, Reclassification of Stranded Tax Effect From AOCI to Retained Earnings $ 784,000    
Coyote Creek Mining Company, L.L.C. (CCMC) [Member] | Otter Tail Power Company [Member] | Lignite Sales Agreement [Member]      
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount   $ 52,200,000  
Variable Interest Entity Reporting Entity Involvement, Maximum Loss Exposure, Percentage   35.00%