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Regulatory Matters (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
Sep. 30, 2018
Dec. 31, 2019
Sep. 30, 2019
Montana Electric Rate Filing [Member]        
Public Utilities, General Disclosures [Line Items]        
Anticipated rate increase, amount   $ 34.9 $ 6.5  
Return on equity, percentage     9.65%  
Change in depreciation resulting from rate review     $ 9.3  
Proposed reduction in return on equity     0.25%  
Approved Interim Rate Increase, Montana Electric Rate Filing. [Member]        
Public Utilities, General Disclosures [Line Items]        
Interim rate increase, amount     $ 10.5  
Deferred revenue, interim rates $ 6.5   2.9  
Power cost and Credit Adjustment Mechanism (PCCAM) [Member]        
Public Utilities, General Disclosures [Line Items]        
Deadband around base electric supply costs     $ 4.1  
Customer sharing percentage of amounts exceeding deadband     90.00%  
Company sharing percentage of amounts exceeding deadband     10.00%  
Under collected supply costs       $ 23.8
Proposed disallowance of replacement power costs   $ 6.0    
Proposed under recovery due to statute change     $ 4.0  
Customer sharing percentage of over/under collection of QF costs     100.00%  
Regulatory assets associated with PCCAM $ 13.2