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Other Events (Details)
$ / shares in Units, t in Millions, $ in Millions, $ in Millions
 1 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Jul. 31, 2018
USD ($)
Jun. 30, 2018
USD ($)
t
Dec. 31, 2020
USD ($)
numberOfMine
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2018
AUD ($)
Dec. 03, 2018
USD ($)
Feb. 06, 2018
Other Commercial Events [Line Items]                    
Number of mines idled during period | numberOfMine         9          
Interest in unincorporated joint venture project         50.00%          
Contribution to construction and development of joint venture         $ 72.0          
Gain on formation of United Wambo Joint Venture         0.0 $ (48.1) $ 0.0      
Revenues         2,881.1 4,623.4 5,581.8      
Provision for equipment loss         0.0 83.2 66.4      
Insurance recovery         0.0 125.0 0.0      
Proceeds from disposal of assets, net of receivables         27.1 30.0 76.4      
Gain on disposition of assets         15.2 2.1 48.2      
DTA and PBGC                    
Other Commercial Events [Line Items]                    
Ownership percentage of equity method investment                   50.00%
Red Mountain Joint Venture                    
Other Commercial Events [Line Items]                    
Proceeds from disposal of assets, net of receivables             20.0      
Gain on disposition of assets             7.1      
Queensland’s Bowen Basin                    
Other Commercial Events [Line Items]                    
Proceeds from disposal of assets, net of receivables               $ 37    
Gain on disposition of assets             20.6      
Shoal Creek                    
Other Commercial Events [Line Items]                    
Customary purchase price adjustments                 $ 389.8  
Purchase price allocation, property, plant, equipment and mine development           365.3        
Purchase price allocation, inventories           39.9        
Purchase price allocation, asset retirement obligations           11.3        
Purchase price allocation, current liabilities           4.1        
Revenues   $ 12.8         6,008.4      
Net income (loss) attributable to common stockholders   $ 0.1                
Excluded acquisition costs             7.4      
Business acquisition, pro forma, income from continuing operations, net of income taxes             $ 826.6      
Basic earnings per share from continuing operations (dollars per share) | $ / shares             $ 5.84      
Diluted earnings per share from continuing operations (dollars per share) | $ / shares             $ 5.75      
Stanmore Coal Limited                    
Other Commercial Events [Line Items]                    
Coal reserves (in tonnes) | t       23            
Proceeds from disposal of assets, net of receivables       $ 22.0            
Gain on disposition of assets     $ 20.5              
Wambo Mine                    
Other Commercial Events [Line Items]                    
Fair value of interest in joint venture         63.7          
Carrying value of net assets         15.6          
North Goonyella Mine                    
Other Commercial Events [Line Items]                    
Costs related to north goonyella events             $ 58.0      
Containment and idling costs         $ 32.3 111.5        
Insurance recovery $ 125.0         125.0        
Deductible amount $ 50.0                  
North Goonyella Mine | Mine Carrying Value                    
Other Commercial Events [Line Items]                    
Costs related to north goonyella events           300.0        
Kayenta Mine                    
Other Commercial Events [Line Items]                    
Consideration held in trust upon termination of coal supply agreement           53.5        
Receivable for additional consideration on settlement, total           78.5        
Receivable for additional consideration on settlement           $ 35.4