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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jul. 31, 2019
Jul. 31, 2018
Defined benefit plans, net pension expense (income) [Abstract]    
Service cost $ 224 $ 233
Interest cost 5,834 6,212
Expected return on plan assets (10,059) (9,902)
Net amortization of prior service cost (19) (24)
Unrecognized net actuarial loss 1,600 1,434
Net pension income (2,420) (2,047)
Employer defined benefit pension plan contributions 4,700 3,600
Expense for employer defined contribution plans $ 4,300 $ 4,500