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Postretirement Health Care and Life Insurance Benefits - Reconciliation of Postretirement Plans' Funded Status to the Balance Sheet (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Postretirement Health Care and Life Insurance Benefits        
Change in benefit obligation:        
Accumulated postretirement benefit obligation at beginning of period $ 659.9 $ 659.9 $ 595.4  
Service cost   3.8 4.8 $ 8.2
Interest cost   20.2 25.1 28.3
Participant contributions   2.4 2.3  
Plan amendments   (185.4) 0.0  
Benefits paid   (42.9) (47.7)  
Actuarial loss   18.6 80.0  
Accumulated postretirement benefit obligation at end of period   476.6 659.9 595.4
Change in plan assets:        
Fair value of plan assets at beginning of period $ 34.2 34.2 15.0  
Actual return on plan assets   3.6 2.2  
Employer contributions   36.4 62.4  
Participant contributions   2.4 2.3  
Benefits paid and administrative fees (net of Medicare Part D reimbursements)   (42.9) (47.7)  
Fair value of plan assets at end of period   33.7 34.2 15.0
Funded status at end of period   (442.9) (625.7)  
Less: Current portion (included in “Accounts payable and accrued expenses”)   29.7 32.3  
Noncurrent obligation (included in “Accrued postretirement benefit costs”)   (413.2) (593.4)  
Postretirement Health Care and Life Insurance Benefits | Retirement Plan Amendment One        
Change in benefit obligation:        
Plan amendments   (174.5)    
Change in plan assets:        
Average remaining service period (in years) 5 years 1 month 6 days      
Postretirement Health Care and Life Insurance Benefits | Retirement Plan Amendment Two        
Change in benefit obligation:        
Plan amendments   $ (10.9)    
Change in plan assets:        
Average remaining service period (in years)   10 years 6 months    
Pension Plan        
Change in benefit obligation:        
Accumulated postretirement benefit obligation at beginning of period $ 853.8 $ 853.8 795.9  
Service cost   0.3 2.0 2.3
Interest cost   28.0 33.5 31.4
Benefits paid   (57.5) (55.6)  
Actuarial loss   (43.4) (78.0)  
Settlement   (51.6) 0.0  
Accumulated postretirement benefit obligation at end of period   816.4 853.8 795.9
Change in plan assets:        
Fair value of plan assets at beginning of period $ 855.2 855.2 764.8  
Actual return on plan assets   101.4 126.0  
Employer contributions   0.0 20.0  
Benefits paid and administrative fees (net of Medicare Part D reimbursements)   (57.5) (55.6)  
Settlement   (51.6) 0.0  
Fair value of plan assets at end of period   847.5 855.2 $ 764.8
Funded status at end of period   31.1 1.4  
Noncurrent obligation (included in “Accrued postretirement benefit costs”)   $ (8.5) $ (12.0)