Earnings Per Share (Computation Of Basic And Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Aug. 31, 2012		May 31, 2012		Feb. 29, 2012		Nov. 30, 2011		Aug. 31, 2011		May 31, 2011		Feb. 28, 2011		Nov. 30, 2010		Aug. 31, 2012		Aug. 31, 2011		Aug. 31, 2010
Earnings Per Share [Abstract]
Net income - attributable to Sonic Corp.	$ 14,502	[1]	$ 14,407	[1]	$ 1,677	[1]	$ 5,499	[1]	$ 12,286	[1]	$ (4,651)	[1]	$ 4,348	[1]	$ 7,242	[1]	$ 36,085		$ 19,225		$ 21,209
Weighted average common shares outstanding - basic																	60,078		61,781		61,319
Effect of dilutive employee stock options and unvested restricted stock units																	94		162		257
Weighted average common shares - diluted																	60,172		61,943		61,576
Net income per common share - basic	$ 0.25	[2]	$ 0.24	[2]	$ 0.03	[2]	$ 0.09	[2]	$ 0.20	[2]	$ (0.08)	[2]	$ 0.07	[2]	$ 0.12	[2]	$ 0.60		$ 0.31		$ 0.35
Net income per common share - diluted	$ 0.25	[2]	$ 0.24	[2]	$ 0.03	[2]	$ 0.09	[2]	$ 0.20	[2]	$ (0.08)	[2]	$ 0.07	[2]	$ 0.12	[2]	$ 0.60		$ 0.31		$ 0.34
Anti-dilutive securities excluded																	6,705	[3]	6,367	[3]	6,834	[3]

[1] Includes a $5.2 million gain and a $28.2 million loss from early extinguishments of debt in the second and third quarters of fiscal year 2011, respectively, and a $1.1 million tax benefit recognized during the first quarter of fiscal year 2011 relating to the favorable settlement of state tax audits.

[2] The sum of per share data may not agree to annual amounts due to rounding.

[3] Anti-dilutive securities consist of stock options and unvested restricted stock units that were not included in the computation of diluted earnings per share because either the exercise price of the options was greater than the average market price of the common stock or the total assumed proceeds under the treasury stock method resulted in negative incremental shares and thus the inclusion would have been anti-dilutive.