Debt (Details Textual) (USD $)								6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2012 Secured Debt	Jun. 30, 2012 Secured Debt TADE	Jun. 30, 2012 Standby Letters of Credit TADE	Jun. 30, 2012 Short term line of credit Line of Credit TADE	Jun. 30, 2012 Long term line of credit	Jun. 30, 2012 Long term line of credit Line of Credit TADE
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	463,400,000
Borrowings under short-term line of credit	309,608,000	71,500,000	194,200,000				1,600,000
Long-term syndicate line								55,000,000
Term loan					12,400,000
Standby letters of credit						822,000
Borrowings outstanding on the long term line of credit									$ 15,000,000