Note 7 - Retained Earnings and Dividend Restriction (Details Textual) - USD ($)		6 Months Ended
	May 01, 2018	Jun. 30, 2018	Dec. 31, 2017
Capitalization, Long-term Debt and Equity, Total		$ 1,304,541,000	$ 1,187,272,000
Otter Tail Power Company [Member]
Equity to Total Capitalization Ratio		52.50%
Net Assets Restricted from Distribution		$ 473,000,000
Capitalization, Long-term Debt and Equity, Total		$ 1,178,024,000
Otter Tail Power Company [Member] | If Approved [Member]
Capitalization, Long-term Debt and Equity, Total	$ 1,204,416,000
Otter Tail Power Company [Member] | Minimum [Member]
Equity to Total Capitalization Ratio		47.40%
Otter Tail Power Company [Member] | Minimum [Member] | Minnesota Public Utilities Commission [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	47.90%
Otter Tail Power Company [Member] | Maximum [Member]
Equity to Total Capitalization Ratio		58.00%
Otter Tail Power Company [Member] | Maximum [Member] | Minnesota Public Utilities Commission [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	58.50%