Short-Term and Long-Term Borrowings - Breakdown of Outstanding Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Mar. 27, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Long-term debt	$ 1,047,000		$ 947,000
Unamortized Long-Term Debt Issuance Costs	3,563		3,266
Total Long-Term Debt, Net of Unamortized Debt Issuance Costs	$ 1,043,437		943,734
Guaranteed Senior Notes
Debt Instrument [Line Items]
Rate	3.55%
Long-term debt	$ 80,000		80,000
Series 2007C Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	6.37%
Long-term debt	$ 42,000		42,000
Series 2013A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	4.68%
Long-term debt	$ 60,000		60,000
Series 2019A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.07%
Long-term debt	$ 10,000		10,000
Series 2020A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.22%
Long-term debt	$ 10,000		10,000
Series 2020B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.22%
Long-term debt	$ 40,000		40,000
Series 2021A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	2.74%
Long-term debt	$ 40,000		40,000
Series 2024A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	5.48%
Long-term debt	$ 60,000		60,000
Series 2025A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	5.49%	5.49%
Long-term debt	$ 50,000		0
Series 2007D Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	6.47%
Long-term debt	$ 50,000		50,000
Series 2019B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.52%
Long-term debt	$ 26,000		26,000
Series 2020C Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.62%
Long-term debt	$ 10,000		10,000
Series 2013B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	5.47%
Long-term debt	$ 90,000		90,000
Series 2018A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	4.07%
Long-term debt	$ 100,000		100,000
Series 2019C Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.82%
Long-term debt	$ 64,000		64,000
Series 2020D Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.92%
Long-term debt	$ 15,000		15,000
Series 2021B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.69%
Long-term debt	$ 100,000		100,000
Series 2022A Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	3.77%
Long-term debt	$ 90,000		90,000
Series 2024B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	5.77%
Long-term debt	$ 60,000		60,000
Series 2025B Senior Unsecured Notes
Debt Instrument [Line Items]
Rate	5.98%
Long-term debt	$ 50,000		$ 0

Otter Tail Corp 10-Q