Borrowing Arrangements (Details) (USD $) In Thousands, except Share data, unless otherwise specified					9 Months Ended															9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Senior Credit Facility, net of discount	Dec. 31, 2012 Senior Credit Facility, net of discount	Sep. 30, 2013 Senior Credit Facility, net of discount LIBOR	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America Maximum	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America Minimum	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America Federal funds rate	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America Bank of America prime rate	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America Base Rate	Sep. 30, 2013 Senior Credit Facility, net of discount Bank of America LIBOR for base rate	Sep. 30, 2013 Senior Credit Facility, net of discount Former Credit Agreement	Sep. 30, 2013 Revolving credit facility Bank of America	Sep. 30, 2013 Letter of credit facility Bank of America	Sep. 30, 2013 Term loan facility Bank of America	Sep. 30, 2013 Other obligations	Dec. 31, 2012 Other obligations	Sep. 30, 2013 Various financing arrangements	Sep. 30, 2013 Convertible Debentures
Borrowing arrangements
Total debt	$ 309,537	$ 310,559	$ 307,383	$ 307,939													$ 2,154	$ 2,620	$ 619	$ 0
Less current portion	21,590	21,837
Long-term borrowings, excluding current portion	287,947	288,722
Senior Credit Facility
Maximum borrowing capacity						450,000								200,000	100,000	250,000
Amortization of principal in the first year																22,500
Amortization of principal in the second year																22,500
Amortization of principal in the third year																30,000
Amortization of principal in the fourth year																30,000
Amortization of principal in the fifth year																37,500
Amount borrowed														72,800
Period of total debt to EBITDA ratio						12 months
Variable rate basis					LIBOR				federal funds rate	Bank of America prime rate	base rate	LIBOR
Interest rate margin on variable rate basis (as a percent)									0.50%			1.00%
Total debt to EBITDA ratio that is required to be maintained							4.5
Fixed charge coverage ratio that is required to be maintained								1.25
Termination penalties													0
Weighted average interest rate on senior credit facility (as a percent)			3.70%	3.70%
Weighted average interest rate on outstanding borrowings, not including letters of credit (as a percent)	3.80%	3.90%
Letters of credit outstanding						59,272
Available borrowing capacity						41,407
Redemption price (in dollars per share)																				$ 19.18
Number of shares																				65,375
Debt redemptions																				$ 1,254