Note 3 - Rate and Regulatory Matters (Details Textual)					1 Months Ended			6 Months Ended
	Sep. 28, 2016	Feb. 12, 2015	Nov. 06, 2014	Nov. 12, 2013	Jun. 30, 2016	Dec. 22, 2015	Apr. 21, 2011 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 25, 2016	Nov. 25, 2009 USD ($)
Regulatory Liabilities								$ 86,929,000	$ 85,727,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Current Return on Equity Used in Transmission Rates	10.32%			12.38%	9.70%	10.32%
Additional Incentive Basis Point			0.50%
Expected Percentage of Return on Equity	10.82%
Proposed Reduced Return on Equity Used in Transmission Rates		8.67%		9.15%
Regulatory Liabilities								$ 1,600,000	$ 2,700,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member]
Utility Incentive Percentage in Next Rolling Twelve Months										13.50%
Utility Incentive Percentage in Next Rolling Year Two										12.00%
Utility Incentive Percentage in Next Rolling Year Three										10.00%
Assumed Savings of Utility										1.70%
Expected Rate of Financial Incentive Reduction										50.00%
Number of Customers								2
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Fiscal Year 2016 [Member]
Amount Of Financial Incentive Requested								$ 5,000,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Fiscal Year 2015 [Member]
Financial Incentive Request Approved								4,300,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Fiscal Year 2014 [Member]
Financial Incentive Request Approved								$ 3,000,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | The 2016 General Rate Case [Member] | Maximum [Member]
Public Utilities Approved Rate Rate of Return on Rate Base Percentage Decrease								8.61%
Public Utilities Return on Equity Percentage Decrease								10.74%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | The 2016 General Rate Case [Member] | Minimum [Member]
Public Utilities Approved Rate Rate of Return on Rate Base Percentage Decrease								7.5056%
Public Utilities Return on Equity Percentage Decrease								9.41%
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2010 General Rate Case [Member]
General Rate Revenue Increase Requested											$ 3,600,000
Percentage of Allowed Rate of Return on Rate Base											8.62%
Percentage of Allowed Rate of Return on Equity											10.75%
Percentage of Increase in Base Rate Revenue Requested											3.00%
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member] | The 2010 General Rate Case [Member]
Public Utilities General Rate Revenue Increase Approved							$ 643,000
Percentage of Increase in Base Rate Revenue Requested							2.32%
Public Utilities Allowed Rate of Return on Rate Base Subsequent to Approval of Increase in Base Rate							8.50%
Otter Tail Power Company [Member] | Big Stone South - Brookings MVP [Member]
Expanded Capacity of Projects								345
Extended Distance of Transmission Line								70
Current Projected Cost								$ 66,300,000
Percentage of Assets of Project								100.00%
Otter Tail Power Company [Member] | Big Stone South - Ellendale MVP [Member] | Federal Energy Regulatory Commission [Member]
Expanded Capacity of Projects								345
Extended Distance of Transmission Line								163
Current Projected Cost								$ 65,500,000
Percentage of Assets of Project								100.00%