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Discontinued Operations - Patriot Related Matters (Details)
 3 Months Ended 6 Months Ended
Jan. 25, 2017
USD ($)
Jun. 30, 2020
USD ($)
retirees
Jun. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
retirees
Jun. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Oct. 09, 2015
buyers
Dec. 31, 2007
USD ($)
Oct. 31, 2007
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                  
Number of Retirees | retirees   0   0          
Estimated fund obligation                 $ 40,000,000
Loss from discontinued operations, net of income taxes   $ (2,300,000) $ (3,400,000) $ (4,500,000) $ (6,800,000)        
Amount contributed to the Combined Benefit Fund   400,000 500,000 800,000 1,000,000.0        
CombinedBenefitFundLowerEstimate   1,000,000              
CombinedBenefitFundFutureEstimate   2,000,000              
Combined Benefit Fund Liability   14,700,000   14,700,000   $ 15,200,000      
Combined benefit fund [Member]                  
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                  
Loss from discontinued operations, net of income taxes   100,000 $ 100,000 200,000 $ 300,000        
Patriot                  
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                  
Number of Buyers | buyers             2    
Spinoff | Patriot | Black Lung Occupational Disease Liability                  
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                  
Initially Determined Potential Exposure from Patriot Bankruptcy               $ 150,000,000  
Potential Exposure from Patriot Bankruptcy   85,700,000   85,700,000   85,700,000      
Spinoff | UMWA 1974 Pension Plan (UMWA Plan) Litigation | Patriot                  
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                  
Settlement of claim, amount awarded to other party $ 75,000,000                
Litigation settlement, liability   $ 12,500,000   $ 12,500,000   $ 26,000,000.0