Note 9 - Commitments and Contingencies (Details Textual) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Loss Contingency, Estimate of Possible Loss	$ 1.0
Otter Tail Power Company [Member]
Number of Utilities	200
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation	$ 1.6	$ 2.7
Otter Tail Power Company [Member] | Construction Programs [Member]
Long-term Purchase Commitment, Amount	$ 71.7	$ 84.8
Otter Tail Power Company [Member] | Coal Purchase Commitments 1 [Member]
Contract Expiration Year	2019
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year	2040
Otter Tail Power Company [Member] | Coal Purchase Commitments [Member]
Long-term Purchase Commitment, Amount	$ 3.0