Pension Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017
Retirement Benefits [Abstract]
Interest cost	$ 781	$ 711	$ 1,562	$ 1,421
Expected return on assets	(720)	(716)	(1,441)	(1,432)
Total net periodic pension cost (benefit)	$ 61	$ (5)	$ 121	$ (11)