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Stockholders' Equity - Accumulated Other Comprehensive Income Loss (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 3 Months Ended
Jul. 31, 2013
Jul. 31, 2012
Jul. 31, 2013
Marketable Securities [Member]
Apr. 30, 2013
Marketable Securities [Member]
Jul. 31, 2013
Commodity Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Beginning balance, April 30, 2013 $ (592)   $ 27 $ 27 $ (619)
Other comprehensive loss before reclassifications         (289)
Amounts reclassified from accumulated other comprehensive loss         164
Other comprehensive loss (125) (2,045)     (125)
Ending balance, July 31, 2013 $ (717)   $ 27 $ 27 $ (744)