Commitments and Contingencies (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Aug. 31, 2015	May. 31, 2015
Commitments and Contingencies (Textual)
Commitments to purchase capital assets	$ 11,500,000	$ 9,500,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Amount outstanding	0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Amount outstanding	$ 0
Line of credit facility, initial term	5 years
Maximum amount outstanding during period	$ 200,000,000
Interest rate on amount borrowed	3.25%
Maximum borrowing capacity	$ 1,000,000,000	$ 750,000,000
Expiration date	Aug. 05, 2020	Jun. 21, 2018
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Amount available	$ 43,000,000	$ 43,000,000
Expiration date range of letters of credit	December 2015 and July 2016
Irrevocable stand by letters of credit amount outstanding	$ 0