Credit Arrangements (Narrative) (Details) (USD $)	3 Months Ended	9 Months Ended			3 Months Ended		3 Months Ended
	May 31, 2012	May 31, 2012	May 31, 2011	May 31, 2012 CMCZ [Member]	May 31, 2012 Revolving credit facility [Member]	Aug. 31, 2011 Revolving credit facility [Member]	May 31, 2012 6.50% notes due July 2017 [Member]	May 31, 2012 7.35% notes due August 2018 [Member]
Debt Instrument [Line Items]
Revolving credit facility expired					$ 400,000,000
Revolving credit facility					300,000,000
Revolving credit facility, maturity date					December 27, 2016
Revolving credit facility, maximum borrowing capacity				71,900,000	400,000,000
Outstanding stand-by letter of credit	26,600,000	26,600,000
Minimum interest coverage ratio for each fiscal quarter on a rolling twelve month through August 31, 2012					250.00%
Minimum interest coverage ratio for for each fiscal quarter on a rolling twelve month cumulative period from November 30, 2012 and beyond					300.00%
Actual interest coverage ratio					459.00%
Maximum debt to capitalization ratio					60.00%
Actual debt to capitalization ratio					52.00%
Amounts outstanding under revolving credit facility				37,000,000	0	0
Notional amount of interest rate derivatives terminated	800,000,000						300,000,000	300,000,000
Net proceeds from termination of interest rate swaps	53,000,000	52,733,000	0
Deferred gain on termination of interest rate swaps	49,800,000	49,800,000
Amortization of deferred gain from termination of interest rate fair value hedge	2,900,000
Short-term debt weighted average interest rate				6.10%
Interest paid		$ 39,800,000	$ 32,600,000