Credit Arrangements (Narrative) (Details) (USD $)	3 Months Ended			6 Months Ended
	Feb. 28, 2013	May 31, 2012	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012
Debt Instrument [Line Items]
Net proceeds from termination of interest rate swaps		$ 53,000,000
Deferred gain on termination of interest rate swaps	41,100,000			41,100,000
Amortization of interest rate swaps termination gain	2,900,000			5,815,000	0
Interest costs capitalized	300,000		100,000	800,000	500,000
Interest paid	33,700,000		30,200,000	38,300,000	34,600,000
CMCP [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	77,100,000			77,100,000
Line of credit facility, amount outstanding	47,400,000			47,400,000
Short-term debt weighted average interest rate	5.40%			5.40%
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				300.00%
Actual interest coverage ratio	564.00%			564.00%
Maximum debt to capitalization ratio				60.00%
Actual debt to capitalization ratio	50.00%			50.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	50,000,000			50,000,000
Stand by letters of credit outstanding amount	$ 29,300,000			$ 29,300,000