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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jan. 31, 2021
Jan. 31, 2020
Jan. 31, 2021
Jan. 31, 2020
Components of net pension income for defined benefit plans [Abstract]        
Service cost $ 356 $ 283 $ 1,036 $ 1,600
Interest cost 4,564 5,919 13,695 18,103
Expected return on plan assets (9,853) (10,217) (28,880) (30,162)
Amortization of prior service cost (23) (18) (72) (56)
Amortization of net actuarial loss 1,944 1,623 5,964 4,804
Net pension income (3,012) (2,410) (8,257) (5,711)
Employer defined benefit pension plan contributions 3,600 3,800 12,500 11,800
Defined Contribution Savings Plans [Abstract]        
Expense for employer defined contribution savings plans $ 5,600 $ 3,200 $ 17,400 $ 10,600