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Environmental Obligations - Narrative (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2021
Mar. 31, 2021
Dec. 31, 2020
Environmental Remediation Obligations [Abstract]      
Environmental loss contingencies, net of expected recoveries, in excess of accrual   $ 170.0  
Pocatello      
Environmental Exit Cost [Line Items]      
Payments for attorney fees and unpaid permit fees   21.4  
Discount   $ 8.9  
Discount rate   2.31%  
Expected aggregate undiscounted fees   $ 84.3 $ 104.5
Charges to expense for new losses   $ 53.5 $ 82.6
Pocatello | Subsequent Event      
Environmental Exit Cost [Line Items]      
Payments for attorney fees and unpaid permit fees $ 9.3