Debt (Details Textual) (USD $)								9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2012 Secured Debt	Sep. 30, 2012 Secured Debt TADE	Sep. 30, 2012 Standby Letters of Credit TADE	Sep. 30, 2012 Short term line of credit Line of Credit TADE	Sep. 30, 2012 Long term line of credit	Sep. 30, 2012 Long term line of credit Line of Credit TADE	Sep. 30, 2012 Secured Debt
Line of Credit Facility [Line Items]
Total borrowing capacity under lines of credit	$ 878,100,000			$ 28,100,000			$ 13,000,000		$ 15,100,000
Total available for borrowings under lines of credit	517,400,000
Borrowings under short-term line of credit	275,522,000	71,500,000	105,000,000				500,000
Long-term syndicate line								55,000,000
Secured long-term debt, noncurrent										19,300,000
Debt instrument, interest rate, stated percentage										3.29%
Term loan					12,400,000
Standby letters of credit						800,000
Borrowings outstanding on the long term line of credit									$ 15,100,000