Note 9 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation	$ 1.6
Otter Tail Power Company [Member] | Construction Programs [Member]
Long-term Purchase Commitment, Amount	$ 77.3	$ 64.5
Contract Expiration Year		2021
Otter Tail Power Company [Member] | Capacity and Energy Requirements [Member]
Contract Expiration Year	2042
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year	2040
Otter Tail Power Company [Member] | Coal Purchase Commitments 3 [Member]
Contract Expiration Year	2020
Otter Tail Power Company [Member] | OTP Land Easements [Member]
Long-term Purchase Commitment, Amount	$ 10.5
Contract Expiration Year	2034
T. O. Plastics, Inc. [Member] | Contract Expiring on December 31, 2021 [Member]
Long-term Purchase Commitment, Amount	$ 4.1	$ 5.0