Long-Term Debt and Capital Lease Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 09, 2011
Long-Term Debt and Capital Lease Obligations
Capital lease obligations	$ 26.7	$ 0
Gross cost of capital leased assets	29.7
Revolving Loan borrowings
Long-Term Debt and Capital Lease Obligations
Maximum borrowing capacity			230.0
Commitment commission (as a percent)	0.375%
Amount of borrowings outstanding	0	0
Revolving Loan borrowings | U.S. dollar denominated loans | Prime rate
Long-Term Debt and Capital Lease Obligations
Borrowing margin (as a percent)	0.75%
Description of variable rate basis	Prime Rate
Revolving Loan borrowings | U.S. dollar denominated loans | Overnight Federal Funds rate
Long-Term Debt and Capital Lease Obligations
Borrowing margin (as a percent)	0.75%
Description of variable rate basis	Overnight Federal Funds Rate
Basis spread on variable rate (as a percent)	0.50%
Revolving Loan borrowings | U.S. dollar denominated loans | Eurodollar rate for one month
Long-Term Debt and Capital Lease Obligations
Borrowing margin (as a percent)	0.75%
Description of variable rate basis	Eurodollar rate for one month
Basis spread on variable rate (as a percent)	1.00%
Revolving Loan borrowings | Eurodollar denominated loans | Eurodollar rate for selected interest period
Long-Term Debt and Capital Lease Obligations
Borrowing margin (as a percent)	1.75%
Description of variable rate basis	Eurodollar rate for the selected interest period
Letter of Credit
Long-Term Debt and Capital Lease Obligations
Maximum borrowing capacity			70.0
Stated interest rate (as a percent)	1.875%
Letters of credit outstanding	$ 33.7	$ 32.0