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Commitments and Contingent Liabilities (Black Lung Benefit Liabilities) (Details)
$ in Millions
12 Months Ended
Feb. 21, 2020
USD ($)
Dec. 31, 2020
USD ($)
claim
Dec. 31, 2019
USD ($)
claim
Dec. 31, 2018
USD ($)
Dec. 31, 2013
USD ($)
Black Lung Benefit          
Defined Benefit Plan Disclosure [Line Items]          
Discount rate   2.00% 2.90%    
Active claims | claim   309 324    
Black lung liability   $ 64.6 $ 55.1    
Increase in black lung expense   1.6      
Current portion of black lung liability   4.6 4.6    
Payments   6.0 5.2 $ 6.3  
Expense (1)   $ 15.4 $ 10.9 $ 5.4  
Uninsured Risk          
Defined Benefit Plan Disclosure [Line Items]          
Payments $ 40.4       $ 8.4