Note 12 - Fair Value of Financial Instruments (Details Textual)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Otter Tail Corporation Credit Agreement [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	1.50%	1.50%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration