Marketable Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended
	Aug. 31, 2012	Aug. 31, 2011	May 31, 2012	Apr. 30, 2012 Conversion option, Kemrock 5.5% bonds	Aug. 31, 2012 Conversion option, Kemrock 5.5% bonds	May 31, 2012 Conversion option, Kemrock 5.5% bonds	Aug. 31, 2012 Maximum	Apr. 30, 2012 Maximum Conversion option, Kemrock 5.5% bonds	May 31, 2012 Maximum Conversion option, Kemrock 5.5% bonds	Apr. 30, 2012 Minimum Conversion option, Kemrock 5.5% bonds	May 31, 2012 Minimum Conversion option, Kemrock 5.5% bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, current	$ 112.7		$ 106.5
Available-for-sale securities, long-term	14.3		13.6
Investment in affiliate				22.7		22.7
Investment in affiliate, interest rate				5.50%	5.50%	5.50%
Debt securities, conversion date								Jun. 12, 2017	Jun. 12, 2017	Jun. 04, 2012	Jun. 04, 2012
Gross gains realized on sales of investments	5.0	1.9
Gross losses realized on sales of investments	0.4	2.8
Losses recognized for securities deemed to have other-than-temporary impairments	$ 0.1	$ 0.4
Investments with unrealized loss, percentage of fair values less than original cost							15.00%