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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Oct. 31, 2021
Oct. 31, 2020
Oct. 31, 2021
Oct. 31, 2020
Components of net pension income for defined benefit plans [Abstract]        
Service cost $ 300 $ 347 $ 607 $ 680
Interest cost 5,178 4,610 10,401 9,131
Expected return on plan assets (10,142) (9,649) (20,401) (19,027)
Amortization of prior service cost (22) (24) (44) (49)
Amortization of net actuarial loss 1,877 2,033 3,774 4,020
Net pension income (2,809) (2,683) (5,663) (5,245)
Employer defined benefit pension plan contributions 4,000 3,800 8,500 8,900
Defined Contribution Savings Plans [Abstract]        
Expense for employer defined contribution savings plans $ 6,500 $ 5,200 $ 15,600 $ 11,800