Quarterly report pursuant to Section 13 or 15(d)

Summary of Marketable Securities (Parenthetical) (Detail)

v3.3.1.900
Summary of Marketable Securities (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Oct. 31, 2015
Oct. 31, 2014
Oct. 31, 2015
Oct. 31, 2014
Apr. 30, 2015
Schedule Of Marketable Securities [Line Items]          
Obligations for which assets are held in trust $ 140,900   $ 140,900   $ 129,100
Gain (loss) on marketable securities $ (2,500) $ 2,500 $ (1,818) $ 4,527