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Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Standard Product Warranty Term		1 year
Standard Product Warranty Accrual, Decrease for Payments		$ 0	$ 0	$ 0
Accounting Standards Update 2018-02 [Member]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	$ 784,000
Otter Tail Power Company [Member] | Coyote Creek Mining Company, L.L.C. (CCMC) [Member] | Lignite Sales Agreement [Member]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		$ 50,400,000
Variable Interest Entity Reporting Entity Involvement, Maximum Loss Exposure, Percentage		35.00%
Electric Plant [Member]
Interest Costs Capitalized		$ 1,728,000	$ 1,206,000	$ 741,000
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service		2.81%	2.76%	2.74%
Electric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life		5 years
Electric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life		82 years
Nonelectric Plant [Member]
Interest Costs Capitalized		$ 0	$ 0	$ 0
Nonelectric Plant [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life		2 years
Nonelectric Plant [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life		40 years

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- Definition

The term of a standard product warranty.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
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-SubTopic 10
-Section 50
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