Credit Arrangements (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Aug. 31, 2012	May 31, 2012	Aug. 31, 2012	Aug. 31, 2011	Aug. 31, 2010	Feb. 29, 2012
Debt Instrument [Line Items]
Revolving credit facility expired						$ 400,000,000
Stand by letters of credit outstanding amount	28,900,000		28,900,000
Notional amount of interest rate derivatives terminated	800,000,000
Net proceeds from termination of interest rate swaps		53,000,000	52,733,000	0	0
Deferred gain on termination of interest rate swaps	46,918,000		46,918,000
Amortization of deferred gain from termination of interest rate fair value hedge	5,800,000
Interest costs capitalized			1,300,000	800,000	4,500,000
Interest paid			74,100,000	71,400,000	80,000,000
CMCZ [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	76,900,000		76,900,000
Line of credit facility, amount outstanding	24,500,000		24,500,000
Short-term debt weighted average interest rate	5.90%		5.90%
Revolving credit facility [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	300,000,000		300,000,000
Revolving credit facility, maturity date	Dec. 27, 2016
Revolving credit facility, maximum borrowing capacity	400,000,000		400,000,000
Minimum interest coverage ratio for the twelve month cumulative period ended August 31, 2012	250.00%
Minimum interest coverage ratio for the twelve month cumulative period ending November 30, 2012 and for each fiscal quarter on a rolling twelve month cumulative period thereafter	300.00%
Actual interest coverage ratio	515.00%		515.00%
Maximum debt to capitalization ratio	60.00%
Actual debt to capitalization ratio	51.00%		51.00%
6.50% notes due July 2017 [Member]
Debt Instrument [Line Items]
Notional amount of interest rate derivatives terminated	300,000,000
7.35% notes due August 2018 [Member]
Debt Instrument [Line Items]
Notional amount of interest rate derivatives terminated	$ 300,000,000