Rate and Regulatory Matters (Detail Textuals 3) - USD ($)				1 Months Ended					3 Months Ended	9 Months Ended
	Feb. 12, 2015	Nov. 06, 2014	Nov. 12, 2013	Sep. 28, 2016	Aug. 31, 2016	Dec. 22, 2015	Aug. 31, 2015	Apr. 21, 2011	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jan. 01, 2012
Regulatory Matters [Line Items]
Regulatory Liabilities										$ 85,769,000		$ 80,754,000
Otter Tail Power Company | South Dakota Public Utilities Commission | Environmental Cost Recovery Rider
Regulatory Matters [Line Items]
Annual revenue requesting recovery					$ 2,300,000		$ 2,700,000
Otter Tail Power Company | South Dakota Public Utilities Commission | 2010 General Rate Case
Regulatory Matters [Line Items]
Revenue increase approved by rate authority								$ 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 | Federal Energy Regulatory Commission
Regulatory Matters [Line Items]
Percentage of prudently incurred costs of construction work in progress, authorized for recovery by formula transmission rate													100.00%
Current return on equity used in transmission rates			12.38%			10.32%
Proposed reduced return on equity used in transmission rates	8.67%		9.15%
Additional Incentive Basis Point		50-basis points
Reductions in revenue									$ 100,000	$ 1,300,000	$ 900,000
Expected percentage of return on equity				10.82%						9.70%
Expected percentage of return on equity, description				ROE will be 10.82% (a 10.32% base ROE plus the 0.5% RTO Adder)
Regulatory Liabilities										$ 2,400,000