Divestiture and Discontinued Operations - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2013
Divestitures [Line Items]
Undiscounted purchase consideration Period	5 years
Loss on disposal of discontinued operations, net of tax	$ 378
Undiscounted purchase consideration paid in installments	21 years
Maine Energy Facility [Member]
Divestitures [Line Items]
Undiscounted purchase consideration	6,650
Bio Fuels Assets [Member]
Divestitures [Line Items]
Undiscounted purchase consideration	$ 2,000