Borrowing Arrangements (Details) (USD $) In Thousands, except Share data, unless otherwise specified					6 Months Ended														6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Senior Credit Facility	Dec. 31, 2012 Senior Credit Facility	Jun. 30, 2013 Senior Credit Facility Bank of America	Jun. 30, 2013 Senior Credit Facility Bank of America Maximum	Jun. 30, 2013 Senior Credit Facility Bank of America Minimum	Jun. 30, 2013 Senior Credit Facility Bank of America Federal funds rate	Jun. 30, 2013 Senior Credit Facility Bank of America Bank of America prime rate	Jun. 30, 2013 Senior Credit Facility Bank of America LIBOR	Jun. 30, 2013 Senior Credit Facility Bank of America Base Rate	Jun. 30, 2013 Senior Credit Facility Bank of America LIBOR for base rate	Jun. 30, 2013 Senior Credit Facility Former Credit Agreement	Jun. 30, 2013 Revolving credit facility Bank of America	Jun. 30, 2013 Letter of credit facility Bank of America	Jun. 30, 2013 Term loan facility Bank of America	Jun. 30, 2013 Other obligations	Dec. 31, 2012 Other obligations	Jun. 30, 2013 Convertible Debentures	Jun. 30, 2013 Various financing arrangements
Borrowing arrangements
Total debt	$ 306,387	$ 310,474	$ 304,191	$ 307,854													$ 2,196	$ 2,620	$ 1,254	$ 942
Less current portion	21,559	21,752
Long-term borrowings, excluding current portion	284,828	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								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)			3.60%	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					60,396
Redemption price (in dollars per share)																			$ 19.18
Number of shares																			65,308
Debt redemptions																			$ 942