Commitments and Contingencies (Details Textual) (USD $)	3 Months Ended
	Aug. 31, 2013	May 31, 2013
Commitments and Contingencies (Textual)
Commitments to purchase capital assets	$ 7,700,000	$ 9,900,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Amount outstanding	0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum amount outstanding during period	25,000,000
Interest rate on amount borrowed	3.25%
Maximum borrowing capacity	500,000,000
Expiration date	Jun. 21, 2018
Amount outstanding	0
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Amount available	35,500,000	36,800,000
Expiration date range of letters of credit	December 2013 and July 2014
Irrevocable stand by letters of credit amount outstanding	$ 0