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Commitments and Contingencies - Schedule of Environmental Liability Associated with Postdam (Detail) (Potsdam Environmental Remediation Liability [Member], USD $)
In Thousands, unless otherwise specified
6 Months Ended 6 Months Ended
Oct. 31, 2014
Oct. 31, 2013
Apr. 30, 2013
Oct. 31, 2013
Obligations Incurred [Member]
Oct. 31, 2014
Accretion Expense [Member]
Oct. 31, 2013
Accretion Expense [Member]
Site Contingency [Line Items]            
Beginning balance $ 5,320 $ 5,397 $ 5,297      
Obligations incurred / Accretion expense       31 69 69
Payments (270)          
Ending balance $ 5,119 $ 5,397 $ 5,297