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Financing Activities (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Jun. 30, 2021
Mar. 31, 2021
Equity Distribution Agreement    
Debt Instrument [Line Items]    
Common Stock Aggregate Gross Sales Price Maximum $ 200.0  
Issuance of shares 879,309  
Common Stock Average Share Price $ 64.91  
Stock Issued, Value, Net of Fees $ 56.3  
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions $ 0.8  
Term Loan Due 2021 [Member]    
Debt Instrument [Line Items]    
Extinguishment of Debt, Amount   $ 100.0
Montana First Mortgage Bonds    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 1.00%  
Debt Instrument, Maturity Date Mar. 26, 2024  
Long-term Debt $ 100.0