Credit Arrangements (Narrative) (Details) (USD $)	12 Months Ended						1 Months Ended			12 Months Ended
	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2011	Aug. 31, 2013 CMCP [Member]	Aug. 31, 2013 Revolving credit facility [Member]	Aug. 31, 2013 Letter of Credit [Member]	May 31, 2013 $330 million notes at 4.875% due May 2023	Aug. 31, 2013 $330 million notes at 4.875% due May 2023	Aug. 31, 2013 Change of control triggering event	Aug. 31, 2013 $200 million notes at 5.625% due November 2013	Aug. 31, 2012 $200 million notes at 5.625% due November 2013	Aug. 31, 2013 $400 million notes at 6.50% due July 2017	Aug. 31, 2012 $400 million notes at 6.50% due July 2017	Aug. 31, 2013 $500 million notes at 7.35% due August 2018	Aug. 31, 2012 $500 million notes at 7.35% due August 2018
Debt Instrument [Line Items]
Debt instrument, face amount							$ 330,000,000	$ 330,000,000		$ 200,000,000	$ 200,000,000	$ 400,000,000	$ 400,000,000	$ 500,000,000	$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage							4.875%	4.875%		5.625%	5.625%	6.50%	6.50%	7.35%	7.35%
Debt Instrument, Maturity Date							May 15, 2023
Proceeds from Debt, Net of Issuance Costs							325,000,000
Debt Extinguishment Total Cash Outflow										205,300,000
Redemption purchase price to principal amount, percentage								100.00%	101.00%
Loss on debt extinguishment	4,758,000	0	0
Line of credit facility, current borrowing capacity				75,900,000	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	28,300,000
Minimum Interest Coverage Ratio					300.00%
Actual interest coverage ratio					517.00%
Maximum debt to capitalization ratio					60.00%
Actual debt to capitalization ratio					51.00%
Net proceeds from termination of interest rate swaps	0	52,733,000	0
Deferred gain of interest rate swaps	34,448,000	46,900,000
Amortization of interest rate swaps termination gain	12,470,000	5,815,000	0
Annual total borrowing				229,400,000
Annual total payments				254,000,000
Line of credit facility, amount outstanding				0
Interest costs capitalized	1,000,000	1,300,000	800,000
Interest paid	$ 82,500,000	$ 74,100,000	$ 71,400,000