Borrowing Arrangements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Borrowing arrangements
Total debt	$ 304,271	$ 288,666
Less current portion	26,537	24,632
Long-term borrowings, excluding current portion	277,734	264,034
Senior Credit Facility, net of discount
Borrowing arrangements
Total debt	302,320	286,672
Senior Credit Facility
Maximum borrowing capacity	450,000
Period after end of each fiscal year mandatory repayment of principal required to be made	90 days
Repayment of mandatory principal	7,940	0
Available borrowing capacity	46,683
Amount that could be borrowed	8,701
Letters of credit outstanding	56,167
Borrowings excluding debt discount	305,460
Debt discount	3,140
Senior Credit Facility, net of discount | Maximum
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	4.0
Senior Credit Facility, net of discount | Maximum | Period End of Each Fiscal Quarter Ending after 1st July 2014 Through 30th June 2015 [Member]
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	3.75
Senior Credit Facility, net of discount | Maximum | End of each fiscal quarter ending after July 1, 2014 through and including June 30, 2016
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	3.5
Senior Credit Facility, net of discount | Maximum | End of each fiscal quarter ending after July 1, 2016
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	3.25
Senior Credit Facility, net of discount | Minimum
Senior Credit Facility
Fixed charge coverage ratio that is required to be maintained	1.25
Senior Credit Facility, net of discount | Minimum | End of each fiscal quarter ending after July 1, 2014
Senior Credit Facility
Fixed charge coverage ratio that is required to be maintained	1.35
Revolving credit facility
Senior Credit Facility
Maximum borrowing capacity	200,000
Letter of credit facility
Senior Credit Facility
Maximum borrowing capacity	100,000
Term loan facility
Senior Credit Facility
Maximum borrowing capacity	250,000
Other obligations
Borrowing arrangements
Total debt	$ 1,951	$ 1,994