Borrowing Arrangements (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			3 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012
Borrowing arrangements
Total debt	$ 253,400				$ 288,666				$ 253,400	$ 288,666
Less current portion	15,567				24,632				15,567	24,632
Long-term borrowings, excluding current portion	237,833				264,034				237,833	264,034
Senior Credit Facility
Interest Expense	4,033	4,162	4,811	4,809	4,613	4,818	4,763	4,840	17,815	19,034	8,616
Weighted average interest rate on senior credit facility (as a percent)	3.20%				3.80%				3.20%	3.80%
Less: Discount on debt	2,300								2,300
Redemption value of convertible debentures outstanding									1,236	1,254
Senior Credit Facility, net of discount
Borrowing arrangements
Total debt	251,010				286,672				251,010	286,672
Senior Credit Facility
Maximum borrowing capacity	450,000								450,000
Period of total debt to EBITDA ratio									12 months
Weighted average interest rate on senior credit facility (as a percent)	3.20%				3.70%				3.20%	3.70%
Current borrowing capacity	24,430								24,430
Letters of credit outstanding	54,859								54,859
Borrowings excluding debt discount	253,310								253,310
Less: Discount on debt	2,300								2,300
Senior Credit Facility, net of discount | Maximum
Senior Credit Facility
Total debt to EBITDA ratio that is required to be maintained									4.5
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 | LIBOR Loans | LIBOR
Senior Credit Facility
Variable rate basis									LIBOR
Senior Credit Facility, net of discount | Base rate loans | Base Rate
Senior Credit Facility
Variable rate basis									federal funds rate
Interest rate margin on variable rate basis (as a percent)									0.50%
Senior Credit Facility, net of discount | Base rate loans | Prime Rate
Senior Credit Facility
Variable rate basis									Bank of America prime rate
Senior Credit Facility, net of discount | Base rate loans | LIBOR
Senior Credit Facility
Variable rate basis									LIBOR
Interest rate margin on variable rate basis (as a percent)									1.00%
Senior Credit Facility, net of discount | Credit Agreement
Senior Credit Facility
Maximum borrowing capacity	81,391								81,391
Revolving credit facility
Senior Credit Facility
Maximum borrowing capacity	200,000								200,000
Amount borrowed									72,800
Credit facility outstanding	93,850								93,850
Letter of credit facility
Senior Credit Facility
Maximum borrowing capacity	100,000								100,000
Credit facility outstanding	53,449								53,449
Term loan facility
Senior Credit Facility
Maximum borrowing capacity	250,000								250,000
Credit facility outstanding	200,000								200,000
Restated Secured Credit Facility
Senior Credit Facility
Credit facility outstanding	53,449								53,449
Other obligations
Borrowing arrangements
Total debt	2,390				1,994				2,390	1,994
Convertible Debentures
Senior Credit Facility
Redemption price (in dollars per share)									$ 19.18
Debt redemptions	0				0				0	0
Former Credit Agreement
Senior Credit Facility
Losses on the extinguishment of debt related to debt issuance costs												51
Termination penalties									0
Former Credit Agreement | Cash flow hedge | Reclassification
Senior Credit Facility
Interest Expense											$ 693