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Financial Instruments - Summary of Gains and Losses on Interest Rate Swap (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2019
Jan. 31, 2018
Interest Rate Swap        
Derivative [Line Items]        
(Losses) gains recognized in OCI, tax effects $ (309) $ 553 $ (189) $ 609