Forward Contracts Classified as Derivatives - Breakdown of OTP's credit risk standing on forward energy contracts in marked-to-market loss positions (Parenthetical) (Details) (Otter Tail Power Company, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Otter Tail Power Company
Credit Derivatives [Line Items]
Contracts Requiring Cash Deposits if OTP's Credit Falls Below Investment Grade	$ 13,294	[1]	$ 16,058	[1]
Offsetting Gains with Counterparties under Master Netting Agreements	(917)		(416)
Reporting Date Deposit Requirement if Credit Risk Feature Triggered	$ 12,377		$ 15,642

[1] Certain OTP derivative energy contracts contain provisions that require an investment grade credit rating from each of the major credit rating agencies on OTP's debt. If OTP's debt ratings were to fall below investment grade, the counterparties to these forward energy contracts could request the immediate deposit of cash to cover contracts in net liability positions.Contracts Requiring Cash Deposits if OTP's Credit Falls Below Investment Grade $ 13,294 $ 16,058 Offsetting Gains with Counterparties under Master Netting Agreements(917) (416) Reporting Date Deposit Requirement if Credit Risk Feature Triggered $ 12,377 $ 15,642