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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jul. 31, 2020
Jul. 31, 2019
Components of net pension income for defined benefit plans [Abstract]    
Service cost $ 333 $ 224
Interest cost 4,521 5,834
Expected return on plan assets (9,378) (10,059)
Amortization of prior service cost (25) (19)
Amortization of net actuarial loss 1,987 1,600
Net pension income (2,562) (2,420)
Employer defined benefit pension plan contributions 5,100 4,700
Defined Contribution Savings Plans [Abstract]    
Expense for employer defined contribution savings plans $ 6,600 $ 4,300