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Note 10 - Short-term and Long-term Borrowings (Details Textual) - Otter Tail Power Company [Member] - USD ($)
6 Months Ended
Feb. 25, 2020
Jun. 30, 2020
Sep. 12, 2019
The 2019 Note Purchase Agreement [Member]      
Proceeds from Issuance of Debt $ 35,000,000    
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount   10.00%  
Debt Instrument, Redemption Price, Percentage   100.00%  
Interest-bearing Debt, Maximum Percentage of Total Capitalization   60.00%  
Priority Debt to Total Capitalization   20.00%  
The 2019 Note Purchase Agreement [Member] | Senior Unsecured Note [Member[      
Debt Instrument, Face Amount     $ 175,000,000
Series 2019 A Senior Unsecured Note Due October 10, 2029 [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]      
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]      
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]      
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]      
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]      
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]      
Debt Instrument, Face Amount     $ 15,000,000
Debt Instrument, Interest Rate, Stated Percentage     3.92%