Commitments and Contingencies (Detail Textuals) (USD $) In Millions, unless otherwise specified		6 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Otter Tail Power Company Construction and Other Purchase Commitments	Dec. 31, 2012 Otter Tail Power Company Construction and Other Purchase Commitments	Jun. 30, 2013 Otter Tail Power Company Capacity and Energy Requirements Property	Jun. 30, 2013 Otter Tail Power Company Capacity and Energy Requirements Property	Dec. 31, 2012 Otter Tail Power Company Capacity and Energy Requirements	Dec. 31, 2012 Otter Tail Power Company Coal Purchase Commitments
Commitments and Contingencies Disclosure [Line Items]
Long-term Purchase Commitment, Amount		$ 154.3	$ 79.4
Additional coal purchase commitments for 2013				2.5	2.5
Contracts expiration year					2032
Loss contingency, range of possible loss, maximum	2.9
Additional coal purchase commitments for 2014				3.2	3.2
Amount for purchase commitments						$ 170.1	$ 797.0
Term of agreement				25 years
Number of wind turbines				39	39