Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($)	3 Months Ended	6 Months Ended	40 Months Ended
	Nov. 30, 2018	Nov. 30, 2018	Nov. 30, 2018	Mar. 21, 2018	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Authorized amount of stock to be repurchased	$ 100,000,000	$ 100,000,000	$ 100,000,000	$ 50,000,000	$ 50,000,000
Total current Board authorizations	0	0	(38,600,000)
Less repurchases made under these authorizations	$ 61,400,000	$ 61,400,000	$ 61,400,000