Notes Payable and Long-Term Debt - Additional Information (Detail) (USD $)	Feb. 28, 2013 7.750% Senior Notes Due 2016	Aug. 31, 2012 7.750% Senior Notes Due 2016	Feb. 28, 2011 7.750% Senior Notes Due 2016	Feb. 28, 2013 8.250% Senior Notes Due 2018	Aug. 31, 2012 8.250% Senior Notes Due 2018	Aug. 31, 2007 8.250% Senior Notes Due 2018	Feb. 28, 2013 5.625% Senior Notes Due 2020	Aug. 31, 2012 5.625% Senior Notes Due 2020	Feb. 28, 2013 4.700% Senior Notes due 2022	Aug. 31, 2012 4.700% Senior Notes due 2022
Debt Instrument [Line Items]
Senior unsecured notes	$ 312,000,000			$ 400,000,000			$ 400,000,000		$ 500,000,000
Senior unsecured notes, interest rate	7.75%	7.75%	7.75%	8.25%	8.25%	8.25%	5.625%	5.625%	4.70%	4.70%
Estimated fair value of senior notes	$ 358,100,000			$ 482,000,000			$ 431,000,000		$ 516,300,000