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Retirement Plans (Details) - USD ($)
$ in Thousands
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Defined benefit plans, net periodic benefit cost [Abstract]    
Service cost $ 230 $ 252
Interest cost 6,252 7,198
Expected return on plan assets (9,657) (9,375)
Net amortization of prior service cost (25) (25)
Recognized net actuarial loss 1,501 1,362
Net pension income (1,699) (588)
Pension plan contributions 2,800 7,600
Additional contribution to defined contribution plan $ 4,900 $ 4,900