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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jul. 31, 2018
Jul. 31, 2017
Defined benefit plans, net pension expense (income) [Abstract]    
Service cost $ 233 $ 230
Interest cost 6,212 6,252
Expected return on plan assets (9,902) (9,657)
Net amortization of prior service cost (24) (25)
Recognized net actuarial loss 1,434 1,501
Net pension income (2,047) (1,699)
Employer defined benefit pension plan contributions 3,600 2,800
Expense for employer defined contribution plans $ 4,500 $ 4,900