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	$ 39.1		$ 30.5
Available-for-sale securities, long-term	87.9		89.6
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
Reclassification of other current assets into long-term assets					73.6	76.0
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%