Document And Entity Information - USD ($)	12 Months Ended
	Aug. 31, 2017	Oct. 13, 2017	Feb. 28, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	SONIC CORP
Entity Central Index Key	0000868611
Current Fiscal Year End Date	--08-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Aug. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		40,250,461
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,041,378,896