Note 3 - Rate and Regulatory Matters - Summary of Revenues Recorded Under Rate Riders (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues recorded under rate riders		$ 849,350	$ 803,539	$ 779,804
Otter Tail Power Company [Member] | Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Revenues recorded under rate riders	[1]	9,225	12,920	10,724
Otter Tail Power Company [Member] | Minnesota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		2,973	5,795	5,202
Otter Tail Power Company [Member] | Minnesota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		8,148	12,443	10,238
Otter Tail Power Company [Member] | North Dakota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		8,729	7,694	6,609
Otter Tail Power Company [Member] | North Dakota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		9,782	11,089	9,502
Otter Tail Power Company [Member] | North Dakota [Member] | Renewable Resource Adjustment [Member]
Revenues recorded under rate riders		7,620	7,800	8,409
Otter Tail Power Company [Member] | South Dakota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Revenues recorded under rate riders		598	468	583
Otter Tail Power Company [Member] | South Dakota [Member] | Transmission Cost Recovery Rider [Member]
Revenues recorded under rate riders		1,843	1,820	1,290
Otter Tail Power Company [Member] | South Dakota [Member] | Environmental Cost Recovery Rider [Member]
Revenues recorded under rate riders		$ 2,345	$ 2,538	$ 1,967

[1] Includes MNCIP costs recovered in base rates.