Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Short-Term Debt	$ 58,117	$ 42,883
Long-Term Debt	534,828	540,942
Less: Current Maturities net of Unamortized Debt Issuance Costs	42,200	33,201
Unamortized Long-Term Debt Issuance Costs	2,242	2,400
Total Long-Term Debt net of Unamortized Debt Issuance Costs	490,386	505,341
Total Short-Term and Long-Term Debt (with current maturities)	590,703	581,425
Term Loan, LIBOR Plus 0.90%, Due February 5, 2018 [Member]
Long-Term Debt	9,000	15,000
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Senior Unsecured Notes 5.95%, Series A, Due August 20, 2017 [Member]
Long-Term Debt	33,000	33,000
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-Term Debt	30,000	30,000
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [member]
Long-Term Debt	42,000	42,000
Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
North Dakota Development Note, 3.95%, Due April 1, 2018 [member]
Long-Term Debt	67	106
Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	761	836
OTP [Member]
Short-Term Debt	58,000	42,883
Long-Term Debt	445,000	445,000
Less: Current Maturities net of Unamortized Debt Issuance Costs	32,993	32,970
Unamortized Long-Term Debt Issuance Costs	1,742	1,861
Total Long-Term Debt net of Unamortized Debt Issuance Costs	410,265	410,169
Total Short-Term and Long-Term Debt (with current maturities)	501,258	486,022
OTP [Member] | Senior Unsecured Notes 5.95%, Series A, Due August 20, 2017 [Member]
Long-Term Debt	33,000	33,000
OTP [Member] | Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
OTP [Member] | Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]
Long-Term Debt	30,000	30,000
OTP [Member] | Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [member]
Long-Term Debt	42,000	42,000
OTP [Member] | Senior Unsecured Notes 4.68%, Series A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
OTP [Member] | Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
OTP [Member] | Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
Otter Tail Corporation [Member]
Short-Term Debt	117
Long-Term Debt	89,828	95,942
Less: Current Maturities net of Unamortized Debt Issuance Costs	9,207	231
Unamortized Long-Term Debt Issuance Costs	500	539
Total Long-Term Debt net of Unamortized Debt Issuance Costs	80,121	95,172
Total Short-Term and Long-Term Debt (with current maturities)	89,445	95,403
Otter Tail Corporation [Member] | Term Loan, LIBOR Plus 0.90%, Due February 5, 2018 [Member]
Long-Term Debt	9,000	15,000
Otter Tail Corporation [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Otter Tail Corporation [Member] | North Dakota Development Note, 3.95%, Due April 1, 2018 [member]
Long-Term Debt	67	106
Otter Tail Corporation [Member] | Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	$ 761	$ 836