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Costs and Estimated Earnings in Excess of Billings, Net - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended
Oct. 31, 2012
Oct. 31, 2011
Oct. 31, 2012
Oct. 31, 2011
Oct. 31, 2012
Contracts Accounted for under Percentage of Completion
Oct. 31, 2011
Contracts Accounted for under Percentage of Completion
Oct. 31, 2012
Contracts Accounted for under Percentage of Completion
Oct. 31, 2011
Contracts Accounted for under Percentage of Completion
Costs and Estimated Earnings in Excess of Billings, Net [Line Items]                
Revenues $ 17,569 $ 15,055 $ 34,254 $ 30,994 $ 8,400 $ 9,600 $ 17,000 $ 17,900