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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Swap Agreements
$ in Millions
Mar. 31, 2020
USD ($)
T
Dec. 31, 2019
T
Derivative Instruments    
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 | $ $ (70.8)  
2020    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 596,850 830,500
Percentage of projected requirements 60.00% 54.00%
2021    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 614,900 488,900
Percentage of projected requirements 39.00% 30.00%
2022    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 404,300 322,900
Percentage of projected requirements 23.00% 19.00%
2023    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 82,400 82,400
Percentage of projected requirements 5.00% 5.00%