Rate and Regulatory Matters (Detail Textuals 1) - USD ($) $ in Millions	1 Months Ended				9 Months Ended	12 Months Ended
	Feb. 16, 2016	Dec. 21, 2015	Sep. 30, 2015	Apr. 25, 2011	Sep. 30, 2016	Dec. 31, 2015	May 25, 2016	Apr. 14, 2016	Apr. 01, 2016	Jul. 09, 2015
Otter Tail Power Company | Rebuttal Testimony
Regulatory Matters [Line Items]
Percentage of allowed rate of return on equity					10.05%
Estimated Interim Rate Refund					$ 2.3
Otter Tail Power Company | Minnesota Public Utilities Commission | Conservation Improvement Program
Regulatory Matters [Line Items]
Financial incentives recognized during period						$ 4.2
Amount of financial incentive requested									$ 4.3
Percentage Increase In Energy Savings						39.00%
Incentives net benefit, 2017							13.50%
Incentives net benefit, 2018							12.00%
Incentives net benefit, 2019							10.00%
Assumed savings of utility							1.70%
Percentage of reduction in financial incentive							50.00%
Otter Tail Power Company | Minnesota Public Utilities Commission | Conservation Improvement Program | Fiscal Year 2014
Regulatory Matters [Line Items]
Financial incentive request approved										$ 3.0
Otter Tail Power Company | Minnesota Public Utilities Commission | Transmission Cost Recovery Rider
Regulatory Matters [Line Items]
Seeking revenue recovery		$ 7.2	$ 7.8
Otter Tail Power Company | Minnesota Public Utilities Commission | 2010 General Rate Case
Regulatory Matters [Line Items]
General rate revenue increase requested				$ 5.0
Percentage of increase in base rate revenue requested				1.60%
Public utilities allowed rate of return on rate base prior to approval of increase in base rate				8.33%
Public utilities allowed rate of return on rate base subsequent to approval of increase in base rate				8.61%
Public utilities allowed rate of return on equity prior to approval of increase in base rate				10.43%
Public utilities allowed rate of return on equity subsequent to approval of increase in base rate				10.74%
Otter Tail Power Company | Minnesota Public Utilities Commission | 2016 General Rate Case
Regulatory Matters [Line Items]
Public Utilities Allowed Rate Of Return On Rate Base	8.07%
Public Utilities Allowed Rate Of Return On Equity Increase In Base Rate	10.40%
Percentage Of Capital	52.50%
Increase to base rate portion of customer bills								9.56%
MNDOC | Direct Testimony
Regulatory Matters [Line Items]
Percentage of allowed rate of return on equity					8.86%
MNDOC | Rebuttal Testimony
Regulatory Matters [Line Items]
Percentage of allowed rate of return on equity					8.66%
OAG | Direct Testimony
Regulatory Matters [Line Items]
Percentage of allowed rate of return on equity					6.96%
OAG | Rebuttal Testimony
Regulatory Matters [Line Items]
Percentage of allowed rate of return on equity					7.14%