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REGULATORY MATTERS: October 2014 Idaho Settlement Stipulation (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Apr. 30, 2018
Oct. 31, 2014
October 2014 Idaho Settlement Stipulation [Line Items]        
Regulatory Liabilities $ 782,185 $ 764,877    
October 2014 Idaho Settlement Stipulation        
October 2014 Idaho Settlement Stipulation [Line Items]        
Authorized Return on Equity in Rate Case, Minimum       9.50%
Authorized Return on Equity in Rate Case, Mid-point       10.00%
Investment Tax Credits, Maximum, in Rate Case 45,000     $ 45,000
Settlement agreement sharing mechanism        
October 2014 Idaho Settlement Stipulation [Line Items]        
Regulatory Liabilities [1] $ 0 $ 5,025    
IDAHO | October 2014 Idaho Settlement Stipulation        
October 2014 Idaho Settlement Stipulation [Line Items]        
Authorized Return on Equity in Rate Case, Minimum       9.50%
Authorized Return on Equity in Rate Case, Mid-point       10.00%
Authorized Return on Equity in Rate Case, Maximum       10.50%
Investment Tax Credits Maximum In One Year In Rate Case       $ 25,000
Investment Tax Credits, Maximum, in Rate Case       $ 45,000
Percentage to be Shared with Customers       75.00%
Percentage to be Shared with Entity       25.00%
Percent To Be Shared With Customers, Power Cost Adjustment       50.00%
Percent To Be Shared With Customers, Pension Balancing       25.00%
Percentage of target ROE, minimum       95.00%
Percentage of target ROE, maximum       105.00%
IDAHO | May 2018 Tax Reform Settlement Stipulation        
October 2014 Idaho Settlement Stipulation [Line Items]        
Authorized Return on Equity in Rate Case, Minimum     9.40%  
Authorized Return on Equity in Rate Case, Mid-point     10.00%  
Authorized Return on Equity in Rate Case, Maximum     10.50%  
Investment Tax Credits Maximum In One Year In Rate Case     $ 25,000  
Investment Tax Credits, Maximum, in Rate Case     $ 45,000  
Percentage to be Shared with Customers     80.00%  
Percentage to be Shared with Entity     20.00%  
Percent To Be Shared With Customers, Power Cost Adjustment     55.00%  
Percent To Be Shared With Customers, Pension Balancing     25.00%  
Percentage of target ROE, minimum     95.00%  
Percentage of target ROE, maximum     105.00%  
[1] This item is discussed in more detail in this Note 3 - "Regulatory Matters."