Borrowing Arrangements (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Borrowing arrangements
Total debt	$ 288,666				$ 310,559				$ 288,666	$ 310,559
Less current portion	24,632				21,837				24,632	21,837
Long-term borrowings, excluding current portion	264,034				288,722				264,034	288,722
Senior Credit Facility
Interest expense	4,613	4,818	4,763	4,840	5,261	1,093	1,132	1,130	19,034	8,616	4,691
Weighted average interest rate on outstanding borrowings, not including letters of credit (as a percent)	3.80%				3.90%				3.80%	3.90%
Discount on debt	3,553								3,553
Senior Credit Facility, net of discount
Borrowing arrangements
Total debt	286,672				307,939				286,672	307,939
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)	3.70%				3.70%				3.70%	3.70%
Letters of credit outstanding	59,498								59,498
Borrowings excluding debt discount	290,225								290,225
Discount on debt	3,553								3,553
Available borrowing capacity	72,303								72,303
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 | Federal funds 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 | Bank of America 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%
Revolving credit facility
Senior Credit Facility
Maximum borrowing capacity	200,000								200,000
Amount borrowed									72,800
Letter of credit facility
Senior Credit Facility
Maximum borrowing capacity	100,000								100,000
Term loan facility
Senior Credit Facility
Maximum borrowing capacity	250,000								250,000
Other obligations
Borrowing arrangements
Total debt	1,994				2,620				1,994	2,620
Convertible Debentures
Borrowing arrangements
Total debt	1,254				1,254				1,254	1,254
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