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REGULATORY MATTERS: Level 4 (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
May 31, 2020
May 31, 2019
May 31, 2018
Jun. 01, 2018
Apr. 30, 2018
Jun. 01, 2017
Jun. 01, 2016
Oct. 31, 2014
Regulatory Matters                      
Additional ADITC amortization $ 500   $ 1,875                
Revenue from contracts with customers 289,573   293,892                
Idaho Power Company                      
Regulatory Matters                      
Additional ADITC amortization 500   1,875                
IDAHO | Idaho Power Company                      
Regulatory Matters                      
Pre-tax charge for relicensing costs write-down   $ 5,000                  
Idaho Settlement Stipulation - Investment Tax Credits and Sharing Mechanism                      
Regulatory Matters                      
Additional ADITC amortization 500   1,900                
Power cost adjustment mechanism                      
Regulatory Matters                      
Regulatory liabilities                 $ 13,000    
Idaho fixed cost adjustment mechanism                      
Regulatory Matters                      
Annual fixed cost adjustment mechanism deferral                 $ 35,000 $ 28,100  
October 2014 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Minimum authorized return on equity                     9.50%
Maximum additional ADITC amortization in one year                     $ 25,000
Maximum additional ADITC amortization                     $ 45,000
Target authorized return on equity                     10.00%
Maximum authorized return on equity                     10.50%
Percentage to be shared with customers                     75.00%
Percentage to be allocated to Idaho Power                     25.00%
Percent to be shared with customers as a rate reduction                     50.00%
Percent to be shared with customers as pension asset amortization                     25.00%
If approved change in Idaho ROE from rate case | October 2014 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Percentage to be shared with customers                     75.00%
Minimum percentage of target ROE                     95.00%
Percent to be shared with customers as a rate reduction                     50.00%
Percent to be shared with customers as pension asset amortization                     25.00%
Maximum percentage of target ROE                     105.00%
Pre-2016 relicensing costs | IDAHO | Idaho Power Company                      
Regulatory Matters                      
Pre-tax charge for relicensing costs write-down   4,300                  
Post-2016 relicensing costs | IDAHO | Idaho Power Company                      
Regulatory Matters                      
Pre-tax charge for relicensing costs write-down   $ 700                  
Subsequent event | Power cost adjustment mechanism                      
Regulatory Matters                      
Amount of approved rate change           $ 10,600          
Amount of requested rate change         $ 22,600            
Subsequent event | Idaho fixed cost adjustment mechanism                      
Regulatory Matters                      
Annual fixed cost adjustment mechanism deferral             $ 15,700        
Subsequent event | April 2018 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Minimum authorized return on equity               9.40%      
Requested annual amortization of regulatory deferrals         7,400            
Maximum additional ADITC amortization in one year               $ 25,000      
Maximum additional ADITC amortization               $ 45,000      
Target authorized return on equity               10.00%      
Maximum authorized return on equity               10.50%      
Percentage to be shared with customers               80.00%      
Percentage to be allocated to Idaho Power               20.00%      
Percent to be shared with customers as a rate reduction               55.00%      
Percent to be shared with customers as pension asset amortization               25.00%      
Subsequent event | Annual recurring rate reduction | April 2018 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Amount of requested rate change         18,700            
Subsequent event | If approved change in Idaho ROE from rate case | April 2018 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Percentage to be shared with customers               80.00%      
Minimum percentage of target ROE               95.00%      
Percent to be shared with customers as a rate reduction               55.00%      
Percent to be shared with customers as pension asset amortization               25.00%      
Maximum percentage of target ROE               105.00%      
Subsequent event | One-time benefit | April 2018 Idaho Settlement Stipulation | IDAHO                      
Regulatory Matters                      
Amount of requested rate change       $ 2,700 7,800            
Retail revenues                      
Regulatory Matters                      
Revenue from contracts with customers 264,525   270,232                
Retail revenues | Tax reform revenue accrual for future rate reduction                      
Regulatory Matters                      
Revenue from contracts with customers [1] $ (5,000)   $ 0                
Scenario, Forecast | Idaho fixed cost adjustment mechanism                      
Regulatory Matters                      
Amount of approved rate change           $ 6,900          
Amount of requested rate change         $ 19,300            
[1] See Note 3 - "Regulatory Matters" for additional discussion regarding Idaho Power's estimate of benefits resulting from the changes in the federal income tax law that Idaho Power expects to return to customers in the future.