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LONG-TERM DEBT: Maturities of Long-term Debt Level 4 (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
Debt Instrument [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months $ 0    
Long-term Debt, Maturities, Repayments of Principal in Year Two 0    
Long-term Debt, Maturities, Repayments of Principal in Year Three 75,000    
Long-term Debt, Maturities, Repayments of Principal in Year Four 49,800    
Long-term Debt, Maturities, Repayments of Principal in Year Five 19,885    
Long-term Debt, Maturities, Repayments of Principal after Year Five 1,845,660    
First mortgage bonds 3.40 Series due 2020      
Debt Instrument [Line Items]      
First mortgage bonds $ 0 $ 100,000  
Debt Instrument, Interest Rate, Stated Percentage 3.40% 3.40%  
First Mortgage Bonds 2.95 Series Due 2022      
Debt Instrument [Line Items]      
First mortgage bonds $ 0 $ 75,000  
Debt Instrument, Interest Rate, Stated Percentage 2.95% 2.95%  
First Mortgage Bonds 2.50 Series due 2023      
Debt Instrument [Line Items]      
First mortgage bonds $ 75,000 $ 75,000  
Debt Instrument, Interest Rate, Stated Percentage 2.50% 2.50%  
First Mortgage Bonds 1.90 Series L Due July 15 2030      
Debt Instrument [Line Items]      
First mortgage bonds $ 80,000 $ 0  
Debt Instrument, Interest Rate, Stated Percentage 1.90%    
First mortgage bonds 6.00 Series due 2032      
Debt Instrument [Line Items]      
First mortgage bonds $ 100,000 $ 100,000  
Debt Instrument, Interest Rate, Stated Percentage 6.00% 6.00%  
First mortgage bonds 5.50 Series due 2033      
Debt Instrument [Line Items]      
First mortgage bonds $ 70,000 $ 70,000  
Debt Instrument, Interest Rate, Stated Percentage 5.50% 5.50%  
First mortgage bonds 5.50 Series due 2034      
Debt Instrument [Line Items]      
First mortgage bonds $ 50,000 $ 50,000  
Debt Instrument, Interest Rate, Stated Percentage 5.50% 5.50%  
First mortgage bonds 5.875 Series due 2034      
Debt Instrument [Line Items]      
First mortgage bonds $ 55,000 $ 55,000  
Debt Instrument, Interest Rate, Stated Percentage 5.875% 5.875%  
First mortgage bonds 5.30 Series due 2035      
Debt Instrument [Line Items]      
First mortgage bonds $ 60,000 $ 60,000  
Debt Instrument, Interest Rate, Stated Percentage 5.30% 5.30%  
First mortgage bonds 6.30 Series due 2037      
Debt Instrument [Line Items]      
First mortgage bonds $ 140,000 $ 140,000  
Debt Instrument, Interest Rate, Stated Percentage 6.30% 6.30%  
First mortgage bonds 6.25 Series due 2037      
Debt Instrument [Line Items]      
First mortgage bonds $ 100,000 $ 100,000  
Debt Instrument, Interest Rate, Stated Percentage 6.25% 6.25%  
First mortgage bonds 4.85 Series due 2040      
Debt Instrument [Line Items]      
First mortgage bonds $ 100,000 $ 100,000  
Debt Instrument, Interest Rate, Stated Percentage 4.85% 4.85%  
First Mortgage Bonds 4.30 Series Due 2042      
Debt Instrument [Line Items]      
First mortgage bonds $ 75,000 $ 75,000  
Debt Instrument, Interest Rate, Stated Percentage 4.30% 4.30%  
First Mortgage Bonds 4.00 Series due 2043      
Debt Instrument [Line Items]      
First mortgage bonds $ 75,000 $ 75,000  
Debt Instrument, Interest Rate, Stated Percentage 4.00% 4.00%  
First mortgage bonds 3.65% Series due 2045      
Debt Instrument [Line Items]      
First mortgage bonds $ 250,000 $ 250,000  
Debt Instrument, Interest Rate, Stated Percentage 3.65% 3.65%  
First mortgage bonds 4.05 Series due 2046      
Debt Instrument [Line Items]      
First mortgage bonds $ 120,000 $ 120,000  
Debt Instrument, Interest Rate, Stated Percentage 4.05% 4.05%  
First Mortgage Bonds 4.20 K Series due 2048      
Debt Instrument [Line Items]      
First mortgage bonds $ 450,000 $ 220,000  
Debt Instrument, Interest Rate, Stated Percentage 4.20% 4.20% 4.20%
Pollution Control Bonds 1.45 due 2024      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage [1] 1.45% 1.45%  
Pollution Control Bond 1.70 due 2026      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage [1] 1.70% 1.70%  
[1] Humboldt County and Sweetwater County Pollution Control Revenue Bonds are secured by the first mortgage bonds, bringing the total first mortgage bonds outstanding at December 31, 2020, to $1.966 billion. These two bonds were purchased and remarketed in August 2019. See "Long-Term Debt Issuances, Maturities, and Redemptions" below.