Fair Value Measurements and Derivative Instruments (Details 5) (Fuel Price Risk, USD $) In Millions, unless otherwise specified	9 Months Ended												3 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Swaps 2012 mg	Dec. 31, 2011 Swaps 2012 mg	Sep. 30, 2012 Swaps 2013 mg	Dec. 31, 2011 Swaps 2013 mg	Sep. 30, 2012 Swaps 2014 mg	Dec. 31, 2011 Swaps 2014 mg	Sep. 30, 2012 Swaps 2015 mg	Dec. 31, 2011 Swaps 2015 mg	Sep. 30, 2012 Swaps 2016 mg	Dec. 31, 2011 Swaps 2016	Sep. 30, 2012 Options 2013
Derivative disclosures
Fuel Swap Agreements (metric tons)			199,900	738,000	755,300	644,000	564,500	418,000	321,000	284,000	104,370
Percentage of projected requirements			58.00%	55.00%	54.00%	47.00%	40.00%	30.00%	22.00%	20.00%	7.00%	0.00%
Derivative instrument, contractual life	4 years
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)	$ 72.8	$ 78.5
Net cash proceeds received upon termination of fuel swaps													$ 10.7