Rate and Regulatory Matters (Detail Textuals) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) kV mi
Otter Tail Power Company | Big Stone South - Brookings MVP
Regulatory Matters [Line Items]
Project costs incurred to date | $	$ 56.4
Percentage of assets of project	100.00%
Expanded capacity of projects | kV	345
Extended distance of transmission line | mi	70
Otter Tail Power Company | Big Stone South - Ellendale MVP | Minimum
Regulatory Matters [Line Items]
Extended distance of transmission line | mi	160
Otter Tail Power Company | Big Stone South - Ellendale MVP | Maximum
Regulatory Matters [Line Items]
Extended distance of transmission line | mi	170
Otter Tail Power Company | Big Stone South - Ellendale MVP | Federal Energy Regulatory Commission
Regulatory Matters [Line Items]
Project costs incurred to date | $	$ 39.8
Percentage of assets of project	100.00%
Expanded capacity of projects | kV	345
Big Stone AQCS Project BART - compliant AQCS
Regulatory Matters [Line Items]
Project costs incurred to date | $	$ 199.3
Capacity Expansion 2020 | Otter Tail Power Company | Brookings Project
Regulatory Matters [Line Items]
Investment to acquire ownership interest | $	$ 26.3
Percentage of ownership interest acquired in transmission line	4.80%
Distance of transmission line | mi	250
Capacity Expansion 2020 | Otter Tail Power Company | Fargo-Monticello Project
Regulatory Matters [Line Items]
Investment to acquire ownership interest | $	$ 81.5
Percentage of ownership interest acquired in transmission line	14.20%
Distance of transmission line | mi	240
Percentage of assets of project	100.00%