Marketable Securities - Additional Information (Detail) - USD ($) |
3 Months Ended | 6 Months Ended | |||||||
---|---|---|---|---|---|---|---|---|---|
Oct. 31, 2015 |
Oct. 31, 2014 |
Oct. 31, 2015 |
Oct. 31, 2014 |
Apr. 30, 2015 |
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Schedule Of Marketable Securities [Line Items] | |||||||||
Proceeds from maturities of available-for-sale securities | $ 8,100,000 | $ 0 | $ 11,100,000 | $ 2,000,000 | |||||
Trading securities | [1],[2] | 144,905,000 | 144,905,000 | $ 131,399,000 | |||||
Gross unrealized gains | 5,300,000 | 5,300,000 | 8,300,000 | ||||||
Gross unrealized losses | $ 900,000 | $ 900,000 | $ 200,000 | ||||||
Minimum | |||||||||
Schedule Of Marketable Securities [Line Items] | |||||||||
Available-for-sale marketable securities, remaining maturities | 1 month | ||||||||
Maximum | |||||||||
Schedule Of Marketable Securities [Line Items] | |||||||||
Available-for-sale marketable securities, remaining maturities | 2 months | ||||||||
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