Forward Contracts Classified as Derivatives - Information on OTP's credit risk exposure on delivered and marked-to-market forward contracts (Details 4) (Otter Tail Power Company, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 Counterparty	Dec. 31, 2012 Counterparty
Net Credit Risk on Forward Energy Contracts
Derivative [Line Items]
Exposure	$ 1,290	$ 580
Counterparties	3	6
Net Credit Risk to Single Largest Counterparty
Derivative [Line Items]
Exposure	$ 850	$ 285