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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Oct. 31, 2018
Oct. 31, 2017
Oct. 31, 2018
Oct. 31, 2017
Defined benefit plans, net pension expense (income) [Abstract]        
Service cost $ 229 $ 242 $ 462 $ 472
Interest cost 6,169 6,346 12,381 12,598
Expected return on plan assets (9,720) (9,782) (19,622) (19,439)
Net amortization of prior service cost (24) (23) (48) (48)
Unrecognized net actuarial loss 1,474 1,534 2,908 3,035
Net pension income (1,872) (1,683) (3,919) (3,382)
Employer defined benefit pension plan contributions 3,500 2,800 7,100 5,600
Expense for employer defined contribution plans $ 2,800 $ 3,000 $ 7,300 $ 7,900