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Defined Benefit Pension Plans - Pension Expenses (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Oct. 31, 2017
Oct. 31, 2016
Oct. 31, 2017
Oct. 31, 2016
Retirement Benefits [Abstract]        
Service cost $ 0 $ 0 $ 0 $ 0
Interest cost 219 232 438 463
Expected return on plan assets (317) (311) (656) (621)
Recognition of net loss 283 318 566 628
Net periodic pension expense $ 185 $ 239 $ 348 $ 470