Fair Value Measurements and Derivative Instruments (Details 5) (Fuel Price Risk, USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative disclosures
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	$ 4.4	$ 47.2
Swaps | 2013
Derivative disclosures
Fuel Swap Agreements (metric tons)	402,000	755,000
Percentage of projected requirements	60.00%	55.00%
Swaps | 2014
Derivative disclosures
Fuel Swap Agreements (metric tons)	768,000	635,000
Percentage of projected requirements	55.00%	45.00%
Swaps | 2015
Derivative disclosures
Fuel Swap Agreements (metric tons)	581,000	363,000
Percentage of projected requirements	39.00%	25.00%
Swaps | 2016
Derivative disclosures
Fuel Swap Agreements (metric tons)	296,000	104,000
Percentage of projected requirements	20.00%	7.00%
Swaps | 2017
Derivative disclosures
Fuel Swap Agreements (metric tons)	74,000
Percentage of projected requirements	5.00%