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LONG-TERM DEBT: Long-term Debt Level 4 (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Sep. 30, 2019
Dec. 31, 2018
Mar. 31, 2018
Debt Instrument [Line Items]        
Total first mortgage bonds $ 1,665,000   $ 1,665,000  
Pollution control revenue bonds Variable Rate Series 2000 due 2027 4,360   4,360  
Total pollution control revenue bonds 170,460   170,460  
American Falls bond guarantee 19,885   19,885  
Unamortized issuance costs and discounts (18,686)   (20,557)  
Total outstanding debt [1] 1,836,659   1,834,788  
Current maturities of long-term debt (100,000)   0  
Long-Term Debt 1,736,659   1,834,788  
Idaho Power Company        
Debt Instrument [Line Items]        
Long-Term Debt $ 1,736,659   $ 1,834,788  
Effective cost of outstanding debt 4.50%   4.83%  
First mortgage bonds 3.40 Series due 2020        
Debt Instrument [Line Items]        
First mortgage bonds $ 100,000   $ 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 $ 75,000   $ 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 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 $ 220,000   $ 220,000 $ 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 [2] 1.45%   5.15%  
Pollution control revenue bonds 5.15 Series due 2024 [2] $ 49,800   $ 49,800  
Pollution Control Bonds 1.45 due 2024 | Idaho Power Company        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage   5.15%    
Pollution Control Bond 1.70 due 2026        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage [2] 1.70%   5.25%  
Pollution control revenue bonds 5.25 Series due 2026 [2] $ 116,300   $ 116,300  
Pollution Control Bond 1.70 due 2026 | Idaho Power Company        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage   5.25%    
Secured debt including Humboldt County and Sweetwater County pollution control revenue bonds        
Debt Instrument [Line Items]        
Total first mortgage bonds $ 1,831,000      
[1] At December 31, 2019 and 2018, the overall effective cost rate of Idaho Power's outstanding debt was 4.50 percent and 4.83 percent, respectively.

[2] Humboldt County and Sweetwater County Pollution Control Revenue Bonds are secured by the first mortgage, bringing the total first mortgage bonds outstanding at December 31, 2019, to $1.831 billion. These two bonds were purchased and remarketed in August of 2019. See "Long-Term Debt Issuances, Maturities, and Redemptions" below.