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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2019
Jan. 31, 2018
Defined benefit plans, net pension expense (income) [Abstract]        
Service cost $ 226 $ 243 $ 688 $ 715
Interest cost 6,103 6,407 18,484 19,005
Expected return on plan assets (9,638) (9,924) (29,260) (29,363)
Net amortization of prior service cost (24) (24) (72) (72)
Unrecognized net actuarial loss 1,429 1,536 4,337 4,550
Pension plan actuarial loss 0 0 0 21
Net pension income (1,904) (1,762) (5,823) (5,144)
Employer defined benefit pension plan contributions 15,800 2,800 22,900 8,400
Employer discretionary contribution 10,000   10,000  
Expense for employer defined contribution plans $ 2,600 $ 3,400 $ 9,900 $ 11,300