Credit Arrangements (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended				1 Months Ended	12 Months Ended
	Feb. 28, 2014	Feb. 28, 2013	Feb. 28, 2014	Feb. 28, 2013	Aug. 31, 2012	Aug. 31, 2013	Feb. 28, 2014 CMCP [Member]	Feb. 28, 2014 Revolving Credit Facility [Member]	Feb. 28, 2014 Letter of Credit [Member]	Aug. 31, 2013 Letter of Credit [Member]	May 31, 2013 $330 million notes at 4.875% due May 2023 [Member]	Aug. 31, 2013 $330 million notes at 4.875% due May 2023 [Member]	Feb. 28, 2014 $330 million notes at 4.875% due May 2023 [Member]	Feb. 28, 2014 Change of control triggering event	May 31, 2013 $200 million notes at 5.625% due November 2013 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											$ 330,000,000	$ 330,000,000	$ 330,000,000		$ 200,000,000
Debt instrument, interest rate, stated percentage											4.875%	4.875%	4.875%
Debt instrument, maturity date											May 15, 2023
Proceeds from issuance of debt, net of underwriting discounts and debt issuance costs											325,000,000
Debt extinguishment total cash outflow												205.3
Redemption purchase price to principal amount, percentage													100.00%	101.00%
Repayments of Long-term debt			3,143,000	2,402,000
Line of credit facility, current borrowing capacity							68,000,000	300,000,000	50,000,000
Revolving credit facility, maturity date								Dec. 27, 2016
Revolving credit facility, maximum borrowing capacity								400,000,000
Stand by letters of credit outstanding amount									38,600,000	28,300,000
Minimum interest coverage ratio								300.00%
Actual interest coverage ratio								489.00%
Maximum debt to capitalization ratio								60.00%
Actual debt to capitalization ratio								50.00%
Net Proceeds From Termination Of Interest Rate Swaps					52,700,000
Unamortized deferred gain on termination of interest rate swaps	30,600,000		30,600,000			34,400,000
Amortization of interest rate swaps termination gain	1,900,000	2,900,000	3,799,000	5,815,000
Period total borrowing							78,000,000
Period total payments							78,000,000
Interest paid	$ 32,800,000	$ 33,700,000	$ 42,300,000	$ 38,300,000