Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended					0 Months Ended		1 Months Ended	3 Months Ended			0 Months Ended								1 Months Ended	3 Months Ended
	Oct. 31, 2013	Oct. 31, 2014	Jul. 31, 2014	Feb. 19, 2014	May 31, 2012	Feb. 21, 2014 Securities Litigation	Feb. 20, 2014 Securities Litigation	Aug. 31, 2013 Securities Litigation	Oct. 31, 2014 Securities Litigation	Jul. 31, 2014 Securities Litigation	Oct. 31, 2013 Securities Litigation	Oct. 31, 2014 Not Designated as Hedging Instrument Warrant Liability	Feb. 18, 2014 Not Designated as Hedging Instrument Warrant Liability	Feb. 18, 2014 Not Designated as Hedging Instrument Warrant Liability	Oct. 31, 2013 Not Designated as Hedging Instrument Warrant Liability	Oct. 31, 2014 Warrant Not Designated as Hedging Instrument Warrant Liability	Jul. 31, 2014 Warrant Not Designated as Hedging Instrument Warrant Liability	Feb. 19, 2014 Warrant Not Designated as Hedging Instrument Warrant Liability	Feb. 18, 2014 Warrant Not Designated as Hedging Instrument Warrant Liability	Jan. 31, 2014 Call Options Long Agreement	Oct. 31, 2014 Call Options Long Agreement	Oct. 31, 2014 Call Options Short Commodity Derivatives Agreement	Feb. 19, 2014 Oaktree Capital Management, L.P.	May 31, 2012 Oaktree Capital Management, L.P.
Fair Value Measurements [Line Items]
Newly-issued senior notes, face amount																								$ 225,000,000
Warrants to purchase common stock, issued				4,420,859	4,420,859																		4,420,859	4,420,859
Exercise price of warrants, value																							44,200,000
Warrants exercise inducement fee				15,000,000
Derivative commodities																				164	28	72
Derivatives, total notional amount																				300,000
Cash equivalents measured at fair value on recurring basis		0
Settlement in cash								11,000,000
Settlement agreement, common stock issuable						4,450,000	4,450,000	4,450,000
Settlement agreement, common stock issuable fair value							123,300,000		0	0	108,600,000
Loss on stock settlement	23,500,000
Derivatives not designated as hedging instruments under ASC 815															75,100,000	0	0	84,100,000	84,100,000
Convertible Debt														75,000,000
Expected volatility												47.50%	45.00%
Total outstanding debt	593,153,000	648,405,000	647,415,000
Notes Payable, Fair Value Disclosure		$ 231,600,000