Borrowing Arrangements (Details) (USD $) In Thousands, except Share data, unless otherwise specified					3 Months Ended												3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Senior Credit Facility	Dec. 31, 2012 Senior Credit Facility	Mar. 31, 2013 Senior Credit Facility Bank of America	Mar. 31, 2013 Senior Credit Facility Bank of America Maximum	Mar. 31, 2013 Senior Credit Facility Bank of America Minimum	Mar. 31, 2013 Senior Credit Facility Bank of America Federal funds rate	Mar. 31, 2013 Senior Credit Facility Bank of America Bank of America prime rate	Mar. 31, 2013 Senior Credit Facility Bank of America LIBOR	Mar. 31, 2013 Senior Credit Facility Bank of America Base Rate	Mar. 31, 2013 Senior Credit Facility Bank of America LIBOR for base rate	Mar. 31, 2013 Senior Credit Facility Former Credit Agreement	Mar. 31, 2013 Term loan facility Bank of America	Mar. 31, 2013 Other obligations	Dec. 31, 2012 Other obligations	Mar. 31, 2013 Convertible Debentures	Mar. 31, 2013 Various financing arrangements
Borrowing arrangements
Total debt	$ 322,586	$ 310,559	$ 320,198	$ 307,939											$ 2,388	$ 2,620	$ 1,254	$ 1,134
Less current portion	21,680	21,837
Long-term borrowings, excluding current portion	300,906	288,722
Senior Credit Facility
Maximum borrowing capacity					450,000
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								federal funds rate	Bank of America prime rate	LIBOR	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)			3.50%	3.70%
Weighted average interest rate on outstanding borrowings, not including letters of credit (as a percent)	3.60%	3.90%
Letters of credit outstanding					60,486
Available borrowing capacity					53,564
Redemption price (in dollars per share)																	$ 19.18
Number of shares																	65,308
Debt redemptions																	$ 0