Short-Term and Long-Term Borrowings - Breakdown of assignment of consolidated short-term and long-term debt outstanding (Parentheticals) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Unsecured Term Loan - LIBOR plus 0.875%, due June 1, 2014
Debt Instrument [Line Items]
Long-Term Debt, Due Date	Jun. 01, 2014
Description of variable rate basis	LIBOR
Basis spread on variable rate	0.875%
Unsecured Term Loan - LIBOR plus 0.875%, due June 1, 2014 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Due Date	Jun. 01, 2014
Description of variable rate basis	LIBOR
Basis spread on variable rate	0.875%
9.000% Notes, due December 15, 2016
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	9.00%	9.00%
Long-Term Debt, Due Date	Dec. 15, 2016	Dec. 15, 2016
9.000% Notes, due December 15, 2016 | Otter Tail Corporation
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	9.00%	9.00%
Long-Term Debt, Due Date	Dec. 15, 2016	Dec. 15, 2016
Senior Unsecured Notes 5.95%, Series A, due August 20, 2017
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	5.95%	5.95%
Long-Term Debt, Due Date	Aug. 20, 2017	Aug. 20, 2017
Senior Unsecured Notes 5.95%, Series A, due August 20, 2017 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	5.95%	5.95%
Long-Term Debt, Due Date	Aug. 20, 2017	Aug. 20, 2017
Grant County, South Dakota Pollution Control Refunding Revenue Bonds 4.65%, due September 1, 2017
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate		4.65%
Long-Term Debt, Due Date		Sep. 01, 2017
Grant County, South Dakota Pollution Control Refunding Revenue Bonds 4.65%, due September 1, 2017 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate		4.65%
Long-Term Debt, Due Date		Sep. 01, 2017
Senior Unsecured Notes 4.63%, due December 1, 2021
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	4.63%	4.63%
Long-Term Debt, Due Date	Dec. 01, 2021	Dec. 01, 2021
Senior Unsecured Notes 4.63%, due December 1, 2021 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	4.63%	4.63%
Long-Term Debt, Due Date	Dec. 01, 2021	Dec. 01, 2021
Senior Unsecured Notes 6.15%, Series B, due August 20, 2022
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.15%	6.15%
Long-Term Debt, Due Date	Aug. 20, 2022	Aug. 20, 2022
Senior Unsecured Notes 6.15%, Series B, due August 20, 2022 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.15%	6.15%
Long-Term Debt, Due Date	Aug. 20, 2022	Aug. 20, 2022
Mercer County, North Dakota Pollution Control Refunding Revenue Bonds 4.85%, due September 1, 2022
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate		4.85%
Long-Term Debt, Due Date		Sep. 01, 2022
Mercer County, North Dakota Pollution Control Refunding Revenue Bonds 4.85%, due September 1, 2022 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate		4.85%
Long-Term Debt, Due Date		Sep. 01, 2022
Senior Unsecured Notes 6.37%, Series C, due August 20, 2027
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.37%	6.37%
Long-Term Debt, Due Date	Aug. 20, 2027	Aug. 20, 2027
Senior Unsecured Notes 6.37%, Series C, due August 20, 2027 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.37%	6.37%
Long-Term Debt, Due Date	Aug. 20, 2027	Aug. 20, 2027
Senior Unsecured Notes 6.47%, Series D, due August 20, 2037
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.47%	6.47%
Long-Term Debt, Due Date	Aug. 20, 2037	Aug. 20, 2037
Senior Unsecured Notes 6.47%, Series D, due August 20, 2037 | Otter Tail Power Company
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	6.47%	6.47%
Long-Term Debt, Due Date	Aug. 20, 2037	Aug. 20, 2037
Other Obligations - Various up to 3.95%
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	3.95%	3.95%
Other Obligations - Various up to 3.95% | Otter Tail Corporation
Debt Instrument [Line Items]
Long-Term Debt, Interest Rate	3.95%	3.95%