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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Swap Agreements
$ in Millions
Sep. 30, 2018
USD ($)
T
Dec. 31, 2017
USD ($)
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 | $ $ 79.4 $ (23.7)
2018    
Derivative Instruments    
Fuel swap agreements (metric tons) 192,200 673,700
Percentage of projected requirements 54.00% 50.00%
2019    
Derivative Instruments    
Fuel swap agreements (metric tons) 758,600 668,500
Percentage of projected requirements 51.00% 46.00%
2020    
Derivative Instruments    
Fuel swap agreements (metric tons) 609,100 531,200
Percentage of projected requirements 39.00% 36.00%
2021    
Derivative Instruments    
Fuel swap agreements (metric tons) 303,000 224,900
Percentage of projected requirements 18.00% 14.00%
2022    
Derivative Instruments    
Fuel swap agreements (metric tons) 80,500 0
Percentage of projected requirements 5.00% 0.00%