Forward Contracts Classified as Derivatives (Detail Textuals) (Otter Tail Power Company, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013
Derivative [Line Items]
Net unrealized gains on forward contracts	$ 40
Investment grade credit ratings
Derivative [Line Items]
Counterparties	3

Balance of recognized but unrealized net gains (losses) on forward energy contracts.

No definition available.

Name:	ottr_RecognizedButUnrealizedNetGainsLossesOnForwardEnergyContracts
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant