Long-Term Debt and Capital Lease Obligations (Details) (USD $) In Millions, unless otherwise specified				12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Apr. 29, 2009 2009 Credit Facility	Dec. 31, 2012 2009 Credit Facility U.S. dollar denominated loans	Dec. 31, 2012 2009 Credit Facility Eurodollar denominated loans	Dec. 31, 2012 2010 Credit Facility	Apr. 28, 2010 2010 Credit Facility	Dec. 31, 2012 2010 Credit Facility U.S. dollar denominated loans	Dec. 31, 2012 2010 Credit Facility Eurodollar denominated loans	Dec. 31, 2012 2011 Credit Facility	Dec. 09, 2011 2011 Credit Facility	Dec. 31, 2012 2011 Credit Facility Eurodollar rate for one month	Dec. 31, 2012 2011 Credit Facility U.S. dollar denominated loans
Credit Facility
Maximum available for the issuance of letters of credit			$ 80.0				$ 80.0				$ 230.0
Sublimit on the amount of revolving loans			30.0				30.0				70.0
Interest rate on letters of credit issued (as a percent)						2.875%				1.875%
Commitment commission (as a percent)						0.50%				0.375%
Borrowing margin (as a percent)				2.25%	3.25%			1.75%	2.75%			1.75%	0.75%
Base interest rate				The higher of the prime rate or one-half of one percent in excess of the overnight "federal funds" rate	Eurodollar rate			The higher of the prime rate or one-half of one percent in excess of the overnight "federal funds" rate	Eurodollar rate			The Eurodollar rate for one month plus 1.00%	The higher of the prime rate, one-half of one percent in excess of the overnight "federal funds" rate
Long Term Debt and Capital Lease Obligations
Letters of credit outstanding	$ 32.0	$ 68.1