Borrowing Arrangements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Borrowing arrangements
Total debt obligations	$ 277,192	$ 288,666
Less: Current portion under Senior Credit Facility and other debt obligations	28,551	24,632
Long-term borrowings, excluding current portion	248,641	264,034
Obligations under Senior Credit Facility, net of original discount on borrowings
Borrowing arrangements
Total debt obligations	274,986	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	68,638
Amount that could be borrowed	43,419
Letters of credit outstanding	56,212
Borrowings excluding debt discount	277,835
Debt discount	2,849
Obligations under Senior Credit Facility, net of original discount on borrowings | Maximum
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	4.0
Obligations under Senior Credit Facility, net of original discount on borrowings | Maximum | End of each fiscal-quarter ending after July 1, 2014 through and including the quarter ending June 30, 2015
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained	3.75
Obligations under Senior Credit Facility, net of original discount on borrowings | 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
Obligations under Senior Credit Facility, net of original discount on borrowings | 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
Obligations under Senior Credit Facility, net of original discount on borrowings | Minimum
Senior Credit Facility
Fixed charge coverage ratio that is required to be maintained	1.25
Obligations under Senior Credit Facility, net of original discount on borrowings | 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 debt obligations
Borrowing arrangements
Total debt obligations	$ 2,206	$ 1,994