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Fair Value Measurements and Derivative Instruments - Fuel Price Risk (Details) - Fuel Price Risk
$ in Millions
Mar. 31, 2016
USD ($)
T
Dec. 31, 2015
USD ($)
T
Derivative Instruments    
Estimated unrealized net gains associated with cash flow hedges pertaining to fuel swap agreements expected to be reclassified to earnings from accumulated other comprehensive income loss | $ $ 294.7 $ 321.0
Swaps 2016    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 692,000 930,000
Percentage of projected requirements 65.00% 65.00%
Swaps 2017    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 854,000 854,000
Percentage of projected requirements 60.00% 59.00%
Swaps 2018    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 583,000 583,000
Percentage of projected requirements 40.00% 40.00%
Swaps 2019    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 308,000 231,000
Percentage of projected requirements 20.00% 15.00%
Swaps 2020    
Derivative Instruments    
Fuel Swap Agreements (metric tons) 79,000 0
Percentage of projected requirements 5.00% 0.00%