Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Defined benefit plans, net pension expense (income) [Abstract]
Service cost	$ 1,093	$ 229	$ 1,317	$ 462
Interest cost	6,350	6,169	12,184	12,381
Expected return on plan assets	(9,886)	(9,720)	(19,945)	(19,622)
Net amortization of prior service cost	(19)	(24)	(38)	(48)
Unrecognized net actuarial loss	1,581	1,474	3,181	2,908
Net pension income	(881)	(1,872)	(3,301)	(3,919)
Employer defined benefit pension plan contributions	3,300	3,500	8,000	7,100
Expense for employer defined contribution plans	$ 3,100	$ 2,800	$ 7,400	$ 7,300