Selected Quarterly Financial Data (Schedule Of Selected Quarterly Financial Data) (Details) (USD $)	0 Months Ended	3 Months Ended																		12 Months Ended
	Aug. 31, 2013 store	Aug. 31, 2013		May 31, 2013		Feb. 28, 2013		Nov. 30, 2012		Aug. 31, 2012		May 31, 2012		Feb. 29, 2012 store		Nov. 30, 2011		May 31, 2011	Feb. 28, 2011	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2011
Total revenues		$ 158,802,000	[1]	$ 146,634,000	[1]	$ 111,141,000	[1]	$ 126,008,000	[1]	$ 150,940,000	[1]	$ 149,427,000	[1]	$ 115,084,000	[1]	$ 128,279,000	[1]			$ 542,585,000	$ 543,730,000	$ 545,951,000
Income from operations		30,033,000		29,994,000		12,018,000		17,203,000		30,902,000		30,736,000		10,548,000		16,754,000				89,248,000	88,940,000	83,308,000
Net income		12,198,000	[2]	14,793,000	[2]	3,577,000	[2]	6,133,000	[2]	14,502,000	[2]	14,407,000	[2]	1,677,000	[2]	5,499,000	[2]			36,701,000	36,085,000	19,225,000
Basic income per share		$ 0.22	[3]	$ 0.26	[3]	$ 0.06	[3]	$ 0.11	[3]	$ 0.25	[3]	$ 0.24	[3]	$ 0.03	[3]	$ 0.09	[3]			$ 0.65	$ 0.60	$ 0.31
Diluted income per share		$ 0.21	[3]	$ 0.26	[3]	$ 0.06	[3]	$ 0.11	[3]	$ 0.25	[3]	$ 0.24	[3]	$ 0.03	[3]	$ 0.09	[3]			$ 0.64	$ 0.60	$ 0.31
Number of refranchised Company Drive-Ins														34
Gain (loss) from early extinguishment of debt		(3,900,000)				(500,000)												(28,200,000)	5,200,000	(4,443,000)		(23,035,000)
Reinstatement of WOTC and resolution of tax matters						700,000
Provision for impairment of long-lived assets																				1,776,000	764,000	824,000
Loss on closure of Company Drive-Ins		2,400,000																		2,400,000
Number of Company Drive-Ins closed	12
Change in vendor for the Sonic system's new point-of-sale technology [Member]
Provision for impairment of long-lived assets		$ 1,600,000

[1] Revenues were impacted by the refranchising of 34 Company Drive-Ins during the latter part of the Company’s second fiscal quarter of 2012.

[2] Includes losses on early extinguishment of debt of $0.5 million and $3.9 million in the second and fourth quarter of fiscal year 2013, respectively, a tax benefit of $0.7 million from the retroactive reinstatement of the Work Opportunity Tax Credit (“WOTC”) and resolution of income tax matters in the second quarter of fiscal year 2013, as well as a $2.4 million loss on the closure of 12 lower-performing Company Drive-Ins as a result of an assessment in advance of capital expenditures for pending technology initiatives and an impairment charge of $1.6 million related to the write-off of assets associated with a change in the vendor for the Sonic system’s new point-of-sale technology in the fourth quarter of fiscal year 2013.

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