Debt (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Oct. 10, 2013 Credit facility	Sep. 30, 2013 Credit facility	Dec. 31, 2012 Credit facility	Oct. 10, 2013 Letters of credit and bankers' acceptances	Sep. 30, 2013 Letters of credit and bankers' acceptances	Sep. 30, 2013 Term Loans	Dec. 31, 2012 Term Loans	Sep. 30, 2013 Acquisition promissory notes	Dec. 31, 2012 Acquisition promissory notes	Sep. 30, 2013 Other	Dec. 31, 2012 Other
Debt
Maximum borrowing capacity						$ 1,100,000,000	$ 800,000,000		$ 400,000,000	$ 300,000,000	$ 242,500,000
Maximum additional borrowings available at the entity's request subject satisfaction of certain conditions							150,000,000
Debt
Total debt	458,645,000		458,645,000		380,318,000		190,000,000	100,500,000			242,500,000	247,500,000	18,070,000	25,878,000	8,075,000	6,440,000
Current maturities of long-term debt	28,642,000		28,642,000		26,065,000
Long term-debt	430,003,000		430,003,000		354,253,000
Interest income, expense and other financing costs
Interest income	1,432,000	256,000	2,357,000	781,000
Interest expense and other financing costs	(6,012,000)	(4,561,000)	(15,175,000)	(15,184,000)
Interest expense and other financing costs, net	$ (4,580,000)	$ (4,305,000)	$ (12,818,000)	$ (14,403,000)