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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Price Risk
$ in Millions
Sep. 30, 2017
USD ($)
T
Dec. 31, 2016
USD ($)
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 | $ $ 81.7 $ 138.5
Swaps 2017    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 218,600 799,065
Percentage of projected requirements 65.00% 60.00%
Swaps 2018    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 756,700 616,300
Percentage of projected requirements 56.00% 44.00%
Swaps 2019    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 668,500 521,000
Percentage of projected requirements 47.00% 35.00%
Swaps 2020    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 531,200 306,500
Percentage of projected requirements 36.00% 20.00%
Swaps 2021    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 224,900 0
Percentage of projected requirements 14.00% 0.00%