v3.26.1
Long-Term Debt and Other Financing (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Debt Instrument [Line Items]    
Long-term debt, gross $ 5,149,363 $ 5,161,805
Current portion (175,854) (176,019)
Unamortized debt issue costs (35,059) (35,447)
Discount payable (1,067) (1,075)
Long-term debt, net of current portion 4,937,383 4,949,264
First Mortgage Bonds    
Debt Instrument [Line Items]    
Long-term debt, gross 1,400,000 1,400,000
First Mortgage Bonds | 2006 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 200,000 200,000
Weighted average interest rate 6.13%  
First Mortgage Bonds | 2017 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 500,000 500,000
Weighted average interest rate 4.75%  
First Mortgage Bonds | 2025 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 700,000 700,000
Weighted average interest rate 5.85%  
First Mortgage Obilgations    
Debt Instrument [Line Items]    
Long-term debt, gross $ 3,500,989 3,511,920
First Mortgage Obilgations | 2005 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 90,000 90,000
Weighted average interest rate 5.85%  
First Mortgage Obilgations | 2007 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 214,711 216,854
Weighted average interest rate 5.75%  
First Mortgage Obilgations | 2008 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 413,389 413,389
Weighted average interest rate 5.92%  
First Mortgage Obilgations | 2009 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 132,222 132,222
Weighted average interest rate 5.47%  
First Mortgage Obilgations | 2011 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 208,080 208,080
Weighted average interest rate 4.53%  
First Mortgage Obilgations | 2012 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 73,949 73,949
Weighted average interest rate 4.07%  
First Mortgage Obilgations | 2015 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 1,353,420 1,353,420
Weighted average interest rate 4.50%  
First Mortgage Obilgations | 2016 CoBank Note    
Debt Instrument [Line Items]    
Long-term debt, gross $ 66,667 68,333
Weighted average interest rate 4.48%  
First Mortgage Obilgations | 2016 CFC Note    
Debt Instrument [Line Items]    
Long-term debt, gross $ 50,420 51,050
Weighted average interest rate 3.74%  
First Mortgage Obilgations | 2022 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 276,810 276,810
Weighted average interest rate 3.00%  
First Mortgage Obilgations | 2024 CoBank Note    
Debt Instrument [Line Items]    
Long-term debt, gross $ 200,000 200,000
Weighted average interest rate 6.14%  
First Mortgage Obilgations | 2024 Series    
Debt Instrument [Line Items]    
Long-term debt, gross $ 344,033 350,525
Weighted average interest rate 6.22%  
First Mortgage Obilgations | 2007 and 2008 Notes    
Debt Instrument [Line Items]    
Long-term debt, gross $ 4,750 4,750
Weighted average interest rate 5.09%  
First Mortgage Obilgations | 2023 Note    
Debt Instrument [Line Items]    
Long-term debt, gross $ 72,000 72,000
Weighted average interest rate 5.56%  
First Mortgage Obilgations | 2025 RUS Loan    
Debt Instrument [Line Items]    
Long-term debt, gross $ 538 538
Weighted average interest rate 4.33%  
Notes    
Debt Instrument [Line Items]    
Long-term debt, gross $ 171,446 171,617
Notes | Dakota Coal    
Debt Instrument [Line Items]    
Long-term debt, gross 76,928 78,268
Notes | Equipment Notes    
Debt Instrument [Line Items]    
Long-term debt, gross $ 21,446 21,617
Weighted average interest rate 5.00%  
Notes | Equipment Notes | Dakota Coal    
Debt Instrument [Line Items]    
Long-term debt, gross $ 64,345 66,462
Weighted average interest rate 4.92%  
Notes | 2019 Tax-Exempt Bonds    
Debt Instrument [Line Items]    
Long-term debt, gross $ 150,000 150,000
Weighted average interest rate 3.63%  
Notes | Other Debt | Dakota Coal    
Debt Instrument [Line Items]    
Long-term debt, gross $ 12,583 $ 11,806