Annual report pursuant to Section 13 and 15(d)

Changes in Each Component of Accumulated Other Comprehensive (Loss) Income (Parenthetical) (Detail)

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Changes in Each Component of Accumulated Other Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Tax effect on reclassifications of realized net losses $ 1,200 $ 1,100 $ 1,200
Tax effects on unrealized (losses) gains 1,900 $ (2,300) $ (2,300)
Tax effect on unrealized (losses) $ (406)