Summary Of Significant Accounting Policies (Narrative) (Details)	3 Months Ended		12 Months Ended
	Aug. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)	Aug. 31, 2018 USD ($) segment	Aug. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Oct. 01, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | segment			1
Number of reportable segments | segment			1
Total restricted cash	$ 27,500,000		$ 27,500,000
Amount of proceeds to be reinvested	12,800,000		$ 12,800,000
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)	19,598,000	$ 19,736,000	19,598,000	$ 19,736,000
Restricted cash (noncurrent)	7,909,000	42,120,000	$ 7,909,000	42,120,000
Number of reporting units | segment			1
Goodwill impairment	0	$ 0
Advertising and promotion costs			$ 12,600,000	15,800,000	$ 23,400,000
Reimbursed Sonic Brand Fund administration costs, percent			15.00%
Reimbursed Sonic Brand Fund administration costs			$ 5,100,000	5,100,000	5,200,000
Reimbursed Brand Technology Fund administration costs, percent			15.00%
Reimbursed Brand Technology Fund administration costs			$ 6,000,000	$ 5,400,000	$ 2,500,000
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
Scenario, Forecast [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment for deferred revenue						$ 17,400,000