General (Details 10) (USD $) In Millions, unless otherwise specified						6 Months Ended		6 Months Ended				0 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jul. 23, 2014 2014 Credit Facility Subsequent event	Jun. 30, 2014 Revolving loan borrowings	Dec. 31, 2013 Revolving loan borrowings	Jun. 30, 2014 Revolving loan borrowings 2011 Credit Facility	Dec. 09, 2011 Revolving loan borrowings 2011 Credit Facility	Jun. 30, 2014 Revolving loan borrowings 2011 Credit Facility U.S. dollar denominated loans Prime rate	Jun. 30, 2014 Revolving loan borrowings 2011 Credit Facility U.S. dollar denominated loans Overnight Federal Funds rate	Jun. 30, 2014 Revolving loan borrowings 2011 Credit Facility U.S. dollar denominated loans Eurodollar rate for one month	Jun. 30, 2014 Revolving loan borrowings 2011 Credit Facility Eurodollar denominated loans Eurodollar rate for selected interest period	Jul. 23, 2014 Revolving loan borrowings 2014 Credit Facility Subsequent event	Jul. 23, 2014 Revolving loan borrowings 2014 Credit Facility U.S. dollar denominated loans Prime rate Subsequent event	Jul. 23, 2014 Revolving loan borrowings 2014 Credit Facility U.S. dollar denominated loans Overnight Federal Funds rate Subsequent event	Jul. 23, 2014 Revolving loan borrowings 2014 Credit Facility U.S. dollar denominated loans Eurodollar rate for one month Subsequent event	Jul. 23, 2014 Revolving loan borrowings 2014 Credit Facility Eurodollar denominated loans Eurodollar rate for selected interest period Subsequent event	Jun. 30, 2014 Letter of credit	Dec. 31, 2013 Letter of credit	Jun. 30, 2014 Letter of credit 2011 Credit Facility	Dec. 09, 2011 Letter of credit 2011 Credit Facility	Jul. 23, 2014 Letter of credit 2014 Credit Facility Subsequent event	Jul. 23, 2014 Term loan 2014 Credit Facility Subsequent event
Long-Term Debt and Capital Lease Obligations
Maximum borrowing capacity			$ 500.0				$ 230.0					$ 250.0								$ 70.0	$ 70.0	$ 250.0
Borrowing margin (as a percent)								0.75%	0.75%	0.75%	1.75%		0.50%	0.50%	0.50%	1.50%
Description of variable rate basis								Prime Rate	Overnight Federal Funds Rate	Eurodollar rate for one month	Eurodollar rate for the selected interest period		Prime Rate	Overnight Federal Funds Rate	Eurodollar rate for one month	Eurodollar rate for the selected interest period
Basis spread on variable rate (as a percent)									0.50%	1.00%				0.50%	1.00%
Stated interest rate (as a percent)																			1.875%		1.625%
Commitment commission (as a percent)						0.375%						0.20%
Capital lease obligations	24.9	26.7
Letters of credit outstanding																	32.9	33.7
Amount of borrowings outstanding				$ 0	$ 0