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LONG-TERM DEBT: Long-term Debt Level 4 (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]      
Total first mortgage bonds $ 1,575,000 $ 1,575,000  
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  
Milner Dam note guarantee 0 1,064  
Unamortized issuance costs and discounts (19,222) (20,731)  
Total outstanding debt [1] 1,746,123 1,745,678  
Current maturities of long-term debt 0 (1,064)  
Long-Term Debt 1,746,123 1,744,614  
Idaho Power Company      
Debt Instrument [Line Items]      
Long-Term Debt $ 1,746,123 $ 1,744,614  
Effective cost of outstanding debt 4.87% 4.87%  
First mortgage bonds 4.50 Series due 2020      
Debt Instrument [Line Items]      
First mortgage bonds $ 130,000 $ 130,000  
Debt Instrument, Interest Rate, Stated Percentage 4.50% 4.50%  
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 3.65% Series due 2045 | Idaho Power Company      
Debt Instrument [Line Items]      
First mortgage bonds     $ 250,000
Debt Instrument, Interest Rate, Stated Percentage     3.65%
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%  
4.05% Series due 2046 | Idaho Power Company      
Debt Instrument [Line Items]      
First mortgage bonds   $ 120,000  
Debt Instrument, Interest Rate, Stated Percentage   4.05%  
Pollution Control Bonds 5.15 due 2024      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage [2] 5.15% 5.15%  
Pollution control revenue bonds 5.15 Series due 2024 [2] $ 49,800 $ 49,800  
Pollution Control Bond 5.25 due 2026      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage [2] 5.25% 5.25%  
Pollution control revenue bonds 5.25 Series due 2026 [2] $ 116,300 $ 116,300  
Secured debt including Humboldt County and Sweetwater County pollution control revenue bonds      
Debt Instrument [Line Items]      
Total first mortgage bonds $ 1,741,000    
[1] At December 31, 2017 and 2016, the overall effective cost rate of Idaho Power's outstanding debt was 4.87 percent.
[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, 2017, to $1.741 billion.