Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019
Defined benefit plans, net pension expense (income) [Abstract]
Service cost	$ 283	$ 226	$ 1,600	$ 688
Interest cost	5,919	6,103	18,103	18,484
Expected return on plan assets	(10,217)	(9,638)	(30,162)	(29,260)
Net amortization of prior service cost	(18)	(24)	(56)	(72)
Unrecognized net actuarial loss	1,623	1,429	4,804	4,337
Net pension income	(2,410)	(1,904)	(5,711)	(5,823)
Employer defined benefit pension plan contributions	3,800	15,800	11,800	22,900
Discretionary contribution		10,000		10,000
Expense for employer defined contribution plans	$ 3,200	$ 2,600	$ 10,600	$ 9,900