Document And Entity Information - shares	6 Months Ended
	Feb. 28, 2018	Mar. 28, 2018
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-Q
Document Period End Date	Feb. 28, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		37,154,893