Borrowing Arrangements (Details) $ in Thousands		3 Months Ended		6 Months Ended
	Feb. 20, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)
Borrowing arrangements
Total debt		$ 258,684		$ 258,684		$ 253,400
Less: Current portion under Senior Credit Facility and other debt obligations		15,959		15,959		15,567
Total long-term borrowings		242,725		242,725		237,833
Senior Credit Facility
Interest Expense		3,062	$ 4,811	7,105	$ 9,620
Amortization of debt discount and issuance costs				634	$ 115
Senior credit facility, net of discount
Borrowing arrangements
Total debt		256,517		256,517		251,010
Senior Credit Facility
Maximum borrowing capacity		450,000		450,000
Current borrowing capacity		80,501		80,501
Letters of credit outstanding		53,449,000		53,449,000
Borrowings excluding debt discount		258,550		258,550
Less: Discount on debt		2,033		2,033
Available borrowing capacity		80,501		$ 80,501
Credit facility outstanding	$ 147,299
Period after end of each fiscal year mandatory repayment of principal required to be made				90 days
Repayment of mandatory principal							$ 7,940
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
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 | Base rate loans | Base Rate
Senior Credit Facility
Interest rate margin on variable rate basis (as a percent)				0.50%
Senior credit facility, net of discount | Base rate loans | LIBOR
Senior Credit Facility
Interest rate margin on variable rate basis (as a percent)				1.00%
Revolving credit facility
Senior Credit Facility
Maximum borrowing capacity	200,000	200,000		$ 200,000
Aggregate additional principal amount	100,000
Letter of credit facility
Senior Credit Facility
Maximum borrowing capacity	100,000	100,000		100,000
Credit facility outstanding	53,449
Swing-Line Loan Facility
Senior Credit Facility
Maximum borrowing capacity	20,000
Term loan facility
Senior Credit Facility
Maximum borrowing capacity	200,000	250,000		250,000
Restated Secured Credit Facility
Senior Credit Facility
Maximum borrowing capacity	400,000
Amount borrowed	10,400
Amortization of principal in the first year	15,000
Amortization of principal in the second year	15,000
Amortization of principal in the third year	20,000
Amortization of principal in the fourth year	20,000
Amortization of principal in the fifth year	20,000
Amortization of principal in the sixth year	$ 110,000
Other obligations
Borrowing arrangements
Total debt		$ 2,167		$ 2,167		$ 2,390