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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Price Risk
$ in Millions
Mar. 31, 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 | $ $ 138.4 $ 138.5
Swaps 2017    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 599,865 799,065
Percentage of projected requirements 60.00% 60.00%
Swaps 2018    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 616,300 616,300
Percentage of projected requirements 44.00% 44.00%
Swaps 2019    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 521,000 521,000
Percentage of projected requirements 35.00% 35.00%
Swaps 2020    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 381,400 306,500
Percentage of projected requirements 25.00% 20.00%
Swaps 2021    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 75,100 0
Percentage of projected requirements 5.00% 0.00%