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Note 10 - Short-term and Long-term Borrowings (Details Textual) - USD ($) |  | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 10, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Oct. 02, 2019 | Sep. 12, 2019 | Feb. 07, 2018 | Sep. 23, 2016 | Aug. 14, 2013 | Oct. 29, 2012 | Dec. 01, 2011
Short-term Debt, Maximum Amount Outstanding During Period |  | $ 109,200,000 |  |  |  |  |  |  |  |
Short-term Debt, Average Outstanding Amount |  | $ 36,200,000 |  |  |  |  |  |  |  |
Short-term Debt, Weighted Average Interest Rate, at Point in Time |  | 3.20% |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  | $ 340,000,000 |  |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] |  |  |  |  |  |  |  |  |  |
Short-term Debt, Maximum Amount Outstanding During Period |  | 38,900,000 |  |  |  |  |  |  |  |
Short-term Debt, Average Outstanding Amount |  | $ 21,900,000 |  |  |  |  |  |  |  |
Short-term Debt, Weighted Average Interest Rate, at Point in Time |  | 3.80% | 3.80% |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  | $ 170,000,000 |  |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Debt to Total Capitalization Ratio |  | 60.00% |  |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Interest and Dividend Coverage Ratio |  | 1.50 |  |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.50% | 1.50% |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] | Unsecured Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  |  |  |  |  | $ 130,000,000 |
Reduced Line of Credit Facility, Borrowing Capacity by Outstanding Letters of Credit |  | $ 40,000,000 |  |  |  |  |  |  |  |
Otter Tail Corporation Credit Agreement [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  |  |  |  |  | 170,000,000 |
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions |  |  |  |  |  |  |  |  | 290,000,000 |
OTP Credit Agreement [Member] |  |  |  |  |  |  |  |  |  |
Short-term Debt, Average Outstanding Amount |  | $ 14,300,000 |  |  |  |  |  |  |  |
Short-term Debt, Weighted Average Interest Rate, at Point in Time |  | 3.60% | 3.00% |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  | $ 170,000,000 |  | $ 73,200,000 |  |  |  |  |  |
Line of Credit Facility, Maximum Amount Outstanding During Period, by Outstanding Letters of Credit |  | $ 50,000,000 |  |  |  |  |  |  |  |
OTP Credit Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Debt to Total Capitalization Ratio |  | 60.00% |  |  |  |  |  |  |  |
OTP Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.25% | 1.25% |  |  |  |  |  |  |
OTP Credit Agreement [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  |  |  |  |  | 170,000,000 |
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions |  |  |  |  |  |  |  |  | $ 250,000,000 |
OTP Credit Agreement [Member] | Revolving Credit Facility [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Repayments of Debt | $ 69,900,000 |  |  |  |  |  |  |  |  |
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt | $ 100,000,000 |  |  |  |  |  |  |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount | 10.00% |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 100.00% |  |  |  |  |  |  |  |  |
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member] | Senior Unsecured Note [Member[ |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 175,000,000 |  |  |  |  |
Series 2019 A Senior Unsecured Note Due October 10, 2029 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 10,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.07% |  |  |  |  |
Series 2019 B Senior Unsecured Note Due October 10, 2039 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 26,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.52% |  |  |  |  |
Series 2019 C Senior Unsecured Note Due October 10, 2049 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 64,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.82% |  |  |  |  |
Series 2020 A Senior Unsecured Notes Due February 25, 2030 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 10,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.22% |  |  |  |  |
Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 40,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.22% |  |  |  |  |
Series 2020 C Senior Unsecured Notes Due February 25, 2040 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 10,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.62% |  |  |  |  |
Series 2020 D Senior Unsecured Notes Due February 25, 2050 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 15,000,000 |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 3.92% |  |  |  |  |
Series 2018 A senior Unsecured Notes Due February 7, 2048 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  | $ 100,000,000 |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  | 4.07% |  |  |  |
The 2018 Note Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount |  |  |  |  |  | 10.00% |  |  |  |
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid |  |  |  |  |  | 100.00% |  |  |  |
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control |  |  |  |  |  | 100.00% |  |  |  |
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  | $ 80,000,000 |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 3.55% | 3.55% |  |  |  | 3.55% |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount |  |  |  |  |  |  | 10.00% |  |  |
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid |  |  |  |  |  |  | 100.00% |  |  |
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control |  |  |  |  |  |  | 100.00% |  |  |
The 9.000% Notes, due December 15, 2016 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 9.00% |  |  |  |  |  |  |
Repayments of Debt |  |  | $ 52,330,000 |  |  |  |  |  |  |
Series A Senior Unsecured Notes due on February 27, 2029 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  |  | $ 60,000,000 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  | 4.68% |  |
Series B Senior Unsecured Notes due on February 27, 2044 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  |  | $ 90,000,000 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  | 5.47% |  |
Note Purchase Agreement 2013 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount |  |  |  |  |  |  |  | 10.00% |  |
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid |  |  |  |  |  |  |  | 100.00% |  |
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control |  |  |  |  |  |  |  | 100.00% |  |
Priority Debt to Total Capitalization |  | 20.00% |  |  |  |  |  |  |  |
Interest-bearing Debt, Maximum Percentage of Total Capitalization |  | 60.00% |  |  |  |  |  |  |  |
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 4.63% | 4.63% |  |  |  |  |  |  |
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  |  |  |  | $ 140,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  |  | 4.63%
The 2007 Notes [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 122,000,000 |  |  |  |  |  |  |  |
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.15% | 6.15% |  |  |  |  |  |  |
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 30,000,000 |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.15% |  |  |  |  |  |  |  |
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.37% | 6.37% |  |  |  |  |  |  |
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 42,000,000 |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.37% |  |  |  |  |  |  |  |
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.47% | 6.47% |  |  |  |  |  |  |
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 50,000,000 |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 6.47% |  |  |  |  |  |  |  |
The 2011 and 2007 Note Purchase Agreements [Member] |  |  |  |  |  |  |  |  |  |
Priority Debt to Total Capitalization |  | 20.00% |  |  |  |  |  |  |  |
The 2011 and 2007 Note Purchase Agreements [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Debt to Total Capitalization Ratio |  | 60.00% |  |  |  |  |  |  |  |
The 2011 and 2007 Note Purchase Agreements [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Interest and Dividend Coverage Ratio |  | 1.50 |  |  |  |  |  |  |  |
The 2011 and 2007 Note Purchase Agreements [Member] | Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount |  | 10.00% |  |  |  |  |  |  |  |
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid |  | 100.00% |  |  |  |  |  |  |  |
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control |  | 100.00% |  |  |  |  |  |  |  |
The 2016 Note Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |
Priority Debt to Total Capitalization |  | 10.00% |  |  |  |  |  |  |  |

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