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Fair Value Measurements and Derivative Instruments (Fuel Price Risk) (Details) - Fuel Price Risk - Fuel Swaps
$ in Millions
Dec. 31, 2015
USD ($)
T
Dec. 31, 2014
USD ($)
T
Derivative instruments disclosure    
Estimated unrealized net gains associated with cash flow hedges pertaining to fuel swap agreements expected to be reclassified to earnings from other accumulated comprehensive income (loss) | $ $ (321.0) $ (223.1)
2015    
Derivative instruments disclosure    
Fuel Swap Agreements (metric tons) 0 806,000
Percentage of projected requirements 0.00% 58.00%
2016    
Derivative instruments disclosure    
Fuel Swap Agreements (metric tons) 930,000 802,000
Percentage of projected requirements 65.00% 55.00%
2017    
Derivative instruments disclosure    
Fuel Swap Agreements (metric tons) 854,000 525,000
Percentage of projected requirements 59.00% 35.00%
2018    
Derivative instruments disclosure    
Fuel Swap Agreements (metric tons) 583,000 226,000
Percentage of projected requirements 40.00% 15.00%
2019    
Derivative instruments disclosure    
Fuel Swap Agreements (metric tons) 231,000 0
Percentage of projected requirements 15.00% 0.00%