Short-Term and Long-Term Borrowings - Breakdown of assignment of consolidated short-term and long-term debt outstanding (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Short-term borrowings	$ 1,117
Long-Term Debt	437,538	421,860
Less: Current Maturities	182	176
Unamortized Debt Discount	3	4
Total Long-Term Debt	437,353	421,680
Total Short-Term and Long-Term Debt (with current maturities)	438,652	421,856
Unsecured Term Loan - LIBOR plus 0.875%, due June 1, 2014
Debt Instrument [Line Items]
Long-Term Debt	40,900
9.000% Notes, due December 15, 2016
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
Senior Unsecured Notes 5.95%, Series A, due August 20, 2017
Debt Instrument [Line Items]
Long-Term Debt	33,000	33,000
Grant County, South Dakota Pollution Control Refunding Revenue Bonds 4.65%, due September 1, 2017
Debt Instrument [Line Items]
Long-Term Debt		5,065
Senior Unsecured Notes 4.63 Percent Due December 1, 2021
Debt Instrument [Line Items]
Long-Term Debt	140,000	140,000
Senior Unsecured Notes 6.15%, Series B, due August 20, 2022
Debt Instrument [Line Items]
Long-Term Debt	30,000	30,000
Mercer County, North Dakota Pollution Control Refunding Revenue Bonds 4.85%, due September 1, 2022
Debt Instrument [Line Items]
Long-Term Debt		20,070
Senior Unsecured Notes 6.37%, Series C, due August 20, 2027
Debt Instrument [Line Items]
Long-Term Debt	42,000	42,000
Senior Unsecured Notes 6.47%, Series D, due August 20, 2037
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Other Obligations - Various up to 3.95%
Debt Instrument [Line Items]
Long-Term Debt	1,638	1,725
Otter Tail Power Company
Debt Instrument [Line Items]
Short-term borrowings	1,117
Long-Term Debt	335,900	320,135
Less: Current Maturities
Unamortized Debt Discount
Total Long-Term Debt	335,900	320,135
Total Short-Term and Long-Term Debt (with current maturities)	337,017	320,135
Otter Tail Power Company | Unsecured Term Loan - LIBOR plus 0.875%, due June 1, 2014
Debt Instrument [Line Items]
Long-Term Debt	40,900
Otter Tail Power Company | Senior Unsecured Notes 5.95%, Series A, due August 20, 2017
Debt Instrument [Line Items]
Long-Term Debt	33,000	33,000
Otter Tail Power Company | Grant County, South Dakota Pollution Control Refunding Revenue Bonds 4.65%, due September 1, 2017
Debt Instrument [Line Items]
Long-Term Debt		5,065
Otter Tail Power Company | Senior Unsecured Notes 4.63 Percent Due December 1, 2021
Debt Instrument [Line Items]
Long-Term Debt	140,000	140,000
Otter Tail Power Company | Senior Unsecured Notes 6.15%, Series B, due August 20, 2022
Debt Instrument [Line Items]
Long-Term Debt	30,000	30,000
Otter Tail Power Company | Mercer County, North Dakota Pollution Control Refunding Revenue Bonds 4.85%, due September 1, 2022
Debt Instrument [Line Items]
Long-Term Debt		20,070
Otter Tail Power Company | Senior Unsecured Notes 6.37%, Series C, due August 20, 2027
Debt Instrument [Line Items]
Long-Term Debt	42,000	42,000
Otter Tail Power Company | Senior Unsecured Notes 6.47%, Series D, due August 20, 2037
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Varistar Corporation
Debt Instrument [Line Items]
Short-term borrowings
Total Short-Term and Long-Term Debt (with current maturities)
Otter Tail Corporation
Debt Instrument [Line Items]
Short-term borrowings
Long-Term Debt	101,638	101,725
Less: Current Maturities	182	176
Unamortized Debt Discount	3	4
Total Long-Term Debt	101,453	101,545
Total Short-Term and Long-Term Debt (with current maturities)	101,635	101,721
Otter Tail Corporation | 9.000% Notes, due December 15, 2016
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
Otter Tail Corporation | Other Obligations - Various up to 3.95%
Debt Instrument [Line Items]
Long-Term Debt	$ 1,638	$ 1,725