Forward Contracts Classified as Derivatives (Detail Textuals) (Otter Tail Power Company, Investment grade credit ratings)	Jun. 30, 2014 Counterparty
Otter Tail Power Company \| Investment grade credit ratings
Derivative [Line Items]
Counterparties	2

Represents the number of derivative counterparties in credit positions at end of period.

No definition available.

Name:	ottr_NumberOfDerivativeCounterparties
Namespace Prefix:	ottr_
Data Type:	xbrli:nonNegativeIntegerItemType
Balance Type:	na
Period Type:	instant