Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2020	May 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 614,815	$ 542,595
Ending Balance	454,872	614,815
Foreign currency translation gain (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,352)	(1,035)
Other comprehensive income before reclassifications, net of tax	11	(317)
Amounts reclassified from accumulated other comprehensive income		0
Total comprehensive income (loss), net of tax	11	(317)
Ending Balance	(1,341)	(1,352)
Unrealized gain (loss) on marketable securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	83
Other comprehensive income before reclassifications, net of tax	0	33
Amounts reclassified from accumulated other comprehensive income		(116)
Total comprehensive income (loss), net of tax	0	(83)
Ending Balance	0	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,352)	(952)
Other comprehensive income before reclassifications, net of tax	11	(284)
Amounts reclassified from accumulated other comprehensive income		(116)
Total comprehensive income (loss), net of tax	11	(400)
Ending Balance	$ (1,341)	$ (1,352)