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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax $ (26.3) $ (25.7) $ (32.1) $ (27.9)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.6 (0.9)    
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.4 1.6    
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax (28.0) (30.7) (34.1) (35.1)
Other Comprehensive Income (Loss), before Reclassifications, before Tax 0.0 0.0    
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 1.3 2.9    
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax 2.4 6.5 3.3 7.8
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.0 0.0    
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (0.9) (1.3)    
Commodity Contract [Member]        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.6 (0.9)    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent (0.7) (1.5) $ (1.3) $ (0.6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.0 0.0    
Settlement Cost [Member]        
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 4.8 1.5    
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]        
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 4.8 1.5    
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]        
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.0 0.0    
Settlement Cost [Member] | Commodity Contract [Member]        
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax $ 0.0 $ 0.0