Summary Of Significant Accounting Policies (Narrative) (Details)	3 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Aug. 31, 2017 USD ($) segment	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | segment			1
Number of reportable segments | segment			1
Total restricted cash	$ 61,900,000		$ 61,900,000
Threshold to reinvest proceeds in eligible capital expenses, term			12 months
Amount used to pay down fixed-rate debt, threshold	5,000,000		$ 5,000,000
Restricted cash (current)	19,736,000	$ 15,873,000	19,736,000	$ 15,873,000
Restricted cash (noncurrent)	42,120,000	140,000	$ 42,120,000	140,000
Number of reporting units | segment			1
Goodwill impairment	0	0
Advertising and promotion costs			$ 15,800,000	23,400,000	$ 24,500,000
Reimbursed Sonic Brand Fund administration costs, percent			15.00%
Reimbursed Sonic Brand Fund administration costs			$ 5,100,000	5,200,000	$ 5,000,000
Reimbursed Brand Technology Fund administration costs, percent			15.00%
Reimbursed Brand Technology Fund administration costs			$ 5,400,000	2,500,000
Unamortized debt issuance costs	9,405,000	11,334,000	9,405,000	11,334,000
Restricted Cash From Sale of Securitized Real Estate [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash (current)	5,000,000		5,000,000
Restricted cash (noncurrent)	$ 42,000,000		$ 42,000,000
Accounting Standards Update 2015-03 [Member] | Other Noncurrent Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Unamortized debt issuance costs		(11,300,000)		(11,300,000)
Accounting Standards Update 2015-03 [Member] | Long-term Debt [Member]
Summary Of Significant Accounting Policies [Line Items]
Unamortized debt issuance costs		$ 11,300,000		$ 11,300,000
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Income tax rate reduction			1.07%