General (Details 10) (USD $)	9 Months Ended							9 Months Ended				9 Months Ended						9 Months Ended					9 Months Ended							0 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Revolving Loan borrowings	Dec. 31, 2013 Revolving Loan borrowings	Sep. 30, 2014 Letter of Credit	Dec. 31, 2013 Letter of Credit	Sep. 30, 2014 Term Loan	Sep. 30, 2014 2011 Credit Facility Revolving Loan borrowings	Dec. 09, 2011 2011 Credit Facility Revolving Loan borrowings	Sep. 30, 2014 2011 Credit Facility Letter of Credit	Dec. 09, 2011 2011 Credit Facility Letter of Credit	Sep. 30, 2014 2011 Credit Facility U.S. dollar denominated loans Prime rate Revolving Loan borrowings	Sep. 30, 2014 2011 Credit Facility U.S. dollar denominated loans Overnight Federal Funds rate Revolving Loan borrowings	Sep. 30, 2014 2011 Credit Facility U.S. dollar denominated loans Eurodollar rate for one month Revolving Loan borrowings	Sep. 30, 2014 2011 Credit Facility Eurodollar denominated loans Eurodollar rate for selected interest period Revolving Loan borrowings	Sep. 30, 2014 Credit Facility 2014	Jul. 23, 2014 Credit Facility 2014	Sep. 30, 2014 Credit Facility 2014 Revolving Loan borrowings	Jul. 23, 2014 Credit Facility 2014 Revolving Loan borrowings	Sep. 30, 2014 Credit Facility 2014 Letter of Credit	Jul. 23, 2014 Credit Facility 2014 Letter of Credit	Jul. 23, 2014 Credit Facility 2014 Term Loan	Sep. 30, 2014 Credit Facility 2014 Prime rate Revolving Loan borrowings	Sep. 30, 2014 Credit Facility 2014 Prime rate Term Loan	Sep. 30, 2014 Credit Facility 2014 Overnight Federal Funds rate Revolving Loan borrowings	Sep. 30, 2014 Credit Facility 2014 Overnight Federal Funds rate Term Loan	Sep. 30, 2014 Credit Facility 2014 Eurodollar rate for one month Revolving Loan borrowings	Sep. 30, 2014 Credit Facility 2014 Eurodollar rate for one month Term Loan	Sep. 30, 2014 Credit Facility 2014 Eurodollar denominated loans Eurodollar rate for selected interest period	Sep. 30, 2014 Credit Facility 2014 Eurodollar denominated loans Eurodollar rate for selected interest period Term Loan	Sep. 30, 2014 Credit Facility 2014 Eurodollar denominated loans Eurodollar rate for selected interest period Term Loan
Long-Term Debt and Capital Lease Obligations
Maximum borrowing capacity									$ 230,000,000		$ 70,000,000						$ 500,000,000		$ 250,000,000		$ 70,000,000	$ 250,000,000
Borrowing margin (as a percent)												0.75%	0.75%	0.75%	1.75%								0.50%	0.50%	0.50%	0.50%	0.50%	0.50%	1.50%
Basis spread on variable rate (as a percent)													0.50%	1.00%											0.50%	0.50%	1.00%	1.00%
Stated interest rate (as a percent)										1.875%										1.625%
Commitment commission (as a percent)								0.375%										0.20%
Capital lease obligations	24,400,000	26,700,000
Letters of credit outstanding					32,900,000	33,700,000
Amount of borrowings outstanding			0	0
Term of option to extend credit facility																1 year
Proceeds from issuance of debt	250,000,000																													250,000,000
Effective borrowing margin (as a percent)																														1.50%
Description of variable rate basis																														One month interest rate
Effective interest rate (as a percent)																															1.654%
Long-term Debt, Fiscal Year Maturity
2014							3,100,000
2015							12,500,000
2016							15,600,000
2017							25,000,000
2018							25,000,000
2019							$ 168,800,000