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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Oct. 31, 2020
Oct. 31, 2019
Oct. 31, 2020
Oct. 31, 2019
Components of net pension income for defined benefit plans [Abstract]        
Service cost $ 347 $ 1,093 $ 680 $ 1,317
Interest cost 4,610 6,350 9,131 12,184
Expected return on plan assets (9,649) (9,886) (19,027) (19,945)
Amortization of prior service cost (24) (19) (49) (38)
Amortization of net actuarial loss 2,033 1,581 4,020 3,181
Net pension income (2,683) (881) (5,245) (3,301)
Employer defined benefit pension plan contributions 3,800 3,300 8,900 8,000
Defined Contribution Savings Plans [Abstract]        
Expense for employer defined contribution savings plans $ 5,200 $ 3,100 $ 11,800 $ 7,400