Summary of Significant Accounting Policies (Detail Textuals) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($)
Significant Accounting Policies [Line Items]
Acquired goodwill	$ 8.2
Reduction in goodwill	$ 2.2
Coyote Creek Mining Company, L.L.C. (CCMC) | Lignite Sales Agreement | Otter Tail Power Company
Significant Accounting Policies [Line Items]
Amortization period	52 months
Percentage of development period costs, development fees and capital charge incurred by CCMC	35.00%
Amount of unrecovered development period costs, development fees and capital charges incurred by CCMC	$ 61.7
Maximum exposure to loss as a result of involvement with CCMC	$ 61.7