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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Swap Agreements
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
USD ($)
T
Jun. 30, 2021
USD ($)
T
t
Jun. 30, 2020
USD ($)
T
Dec. 31, 2020
T
t
Derivative Instruments        
Discontinued cash flow hedge, nonmonetary amount 100,000 48,000 300,000  
Estimated unrealized net loss associated with cash flow hedges pertaining to fuel swap agreements expected to be reclassified to earnings from accumulated other comprehensive income loss | $   $ 34.5    
Other income (expense)        
Derivative Instruments        
Reclassification | $ $ 13.7 $ 4.4 $ 68.6  
2021        
Derivative Instruments        
Fuel Swap Agreements (metric tons)   214,850   385,050
Percentage of projected requirements   38.00%   40.00%
2021 | Not Designated as Hedging Instrument        
Derivative Instruments        
Fuel Swap Agreements (metric tons) | t   82,500   229,850
2022        
Derivative Instruments        
Fuel Swap Agreements (metric tons)   389,650   389,650
Percentage of projected requirements   25.00%   23.00%
2022 | Not Designated as Hedging Instrument        
Derivative Instruments        
Fuel Swap Agreements (metric tons) | t   14,650   14,650
2023        
Derivative Instruments        
Fuel Swap Agreements (metric tons)   82,400   82,400
Percentage of projected requirements   5.00%   5.00%