Self-insurance (Tables) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 26, 2015	Jun. 27, 2015	Mar. 28, 2015	Sep. 27, 2014	Jun. 28, 2014	Mar. 29, 2014	Jun. 27, 2015	Jun. 28, 2014	Sep. 26, 2015	Sep. 27, 2014	Dec. 27, 2014	Dec. 28, 2013	Dec. 29, 2012	Jan. 01, 2012
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued self-insurance	$ 17,676								$ 17,676		$ 18,676
Deferred income taxes	6,584								6,584		5,816
Total current liabilities	209,524								209,524		210,614
Accrued self-insurance	22,364								22,364		22,364
Deferred income taxes	85,517								85,517		91,464
Total liabilities	339,900								339,900		346,356
Retained earnings	999,387								999,387		980,842
Total shareholders' equity	862,950								862,950		844,763
Total liabilities and shareholders' equity	1,202,850								1,202,850		1,191,119
Operating, general and administrative expenses	174,566			$ 166,067					523,170	$ 499,435
Income from operations	19,581			19,610					65,011	60,683
Income before provision for income taxes	19,159			19,967					65,706	62,449
Provision for income taxes	6,371			6,456					22,952	21,767
Net income	$ 12,788			$ 13,511					$ 42,754	$ 40,682
Basic and diluted earnings per share	$ 0.48			$ 0.50					$ 1.59	$ 1.51
Deferred income taxes									$ (4,929)	$ (42)
Accounts payable and other liabilities									(4,533)	(6,522)
Net cash provided by operating activities									$ 100,540	91,670
Self-Insurance Error Correction [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued self-insurance		$ 18,084	$ 19,215				$ 18,084				18,676	$ 16,460
Deferred income taxes		7,633	5,400				7,633				5,816	5,410
Total current liabilities		202,889	203,103				202,889				210,614	191,538
Accrued self-insurance		22,364	22,364				22,364				22,364	23,882
Deferred income taxes		82,394	88,159				82,394				91,464	88,017
Total liabilities		329,457	337,232				329,457				346,356	326,472
Retained earnings		994,670	986,095				994,670				980,842	958,739
Total shareholders' equity		858,474	850,031				858,474				844,763	821,770
Total liabilities and shareholders' equity		1,187,931	1,187,263				1,187,931				1,191,119	1,148,242
Operating, general and administrative expenses		173,353	175,250	$ 166,067	$ 168,620	$ 164,799	348,603	$ 333,369		499,435	671,587	631,796	$ 614,860
Income from operations		25,566	19,865	19,610	19,104	21,969	45,431	41,073		60,683	81,375	113,672	127,693
Income before provision for income taxes		26,151	20,397	19,967	19,760	22,722	46,548	42,482		62,449	83,662	118,356	131,575
Provision for income taxes		9,507	7,074	6,456	7,158	8,152	16,581	15,311		21,767	29,281	45,170	48,675
Net income		$ 16,644	$ 13,323	$ 13,511	$ 12,602	$ 14,570	$ 29,967	$ 27,171		$ 40,682	$ 54,381	$ 73,186	$ 82,900
Basic and diluted earnings per share		$ 0.62	$ 0.50	$ 0.50	$ 0.47	$ 0.54	$ 1.12	$ 1.01		$ 1.51	$ 2.02	$ 2.72	$ 3.08
Deferred income taxes			$ (3,730)			$ (1,446)	$ (7,169)	$ (4,162)		$ (42)	$ 3,235	$ 11,402	$ 15,883
Accounts payable and other liabilities			(10,030)			(964)	(14,240)	(10,578)		(6,522)	17,230	7,056	(5,706)
Net cash provided by operating activities			31,026			34,192	63,918	55,723		91,670	123,110	142,632	123,961
Self-Insurance Error Correction [Member] | As Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued self-insurance		$ 18,830	19,597				18,830				18,695	19,333
Deferred income taxes		7,318	5,235				7,318				5,800	4,219
Total current liabilities		203,320	203,320				203,320				210,617	193,220
Accrued self-insurance		0	0				0				0	0
Deferred income taxes		91,686	97,451				91,686				100,756	97,934
Total liabilities		316,816	324,377				316,816				333,287	314,189
Retained earnings		1,007,311	998,950				1,007,311				993,911	971,022
Total shareholders' equity		871,115	862,886				871,115				857,832	834,053
Total liabilities and shareholders' equity		1,187,931	1,187,263				1,187,931				1,191,119	1,148,242
Operating, general and administrative expenses		173,717	175,614	$ 165,733	$ 168,286	164,465	349,331	332,701		498,433	670,251	634,286	615,521
Income from operations		25,202	19,501	19,944	19,438	22,303	44,703	41,741		61,685	82,711	111,182	127,032
Income before provision for income taxes		25,787	20,033	20,301	20,094	23,056	45,820	43,150		63,451	84,998	115,866	130,914
Provision for income taxes		9,357	6,924	6,594	7,296	8,290	16,281	15,586		22,180	29,831	44,145	48,403
Net income		$ 16,430	$ 13,109	$ 13,707	$ 12,798	$ 14,766	$ 29,539	$ 27,564		$ 41,271	$ 55,167	$ 71,721	$ 82,511
Basic and diluted earnings per share		$ 0.61	$ 0.49	$ 0.51	$ 0.48	$ 0.55	$ 1.10	$ 1.02		$ 1.53	$ 2.05	$ 2.67	$ 3.07
Deferred income taxes			$ (3,880)			$ (1,308)	$ (7,469)	$ (3,887)		$ 371	$ 3,785	$ 10,377	$ 15,611
Accounts payable and other liabilities			(9,666)			(1,298)	(13,512)	(11,246)		(7,524)	15,894	9,546	(5,045)
Net cash provided by operating activities			31,026			34,192	63,918	55,723		91,670	123,110	142,632	123,961
Self-Insurance Error Correction [Member] | Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued self-insurance		$ (746)	(382)				(746)				(19)	(2,873)
Deferred income taxes		315	165				315				16	1,191
Total current liabilities		(431)	(217)				(431)				(3)	(1,682)
Accrued self-insurance		22,364	22,364				22,364				22,364	23,882
Deferred income taxes		(9,292)	(9,292)				(9,292)				(9,292)	(9,917)
Total liabilities		12,641	12,855				12,641				13,069	12,283
Retained earnings		(12,641)	(12,855)				(12,641)				(13,069)	(12,283)		$ 14,200
Total shareholders' equity		(12,641)	(12,855)				(12,641)				(13,069)	(12,283)
Total liabilities and shareholders' equity		0	0				0				0	0
Operating, general and administrative expenses		(364)	(364)	$ 334	$ 334	334	(728)	668		1,002	1,336	(2,490)	(661)
Income from operations		364	364	(334)	(334)	(334)	728	(668)		(1,002)	(1,336)	2,490	661
Income before provision for income taxes		364	364	(334)	(334)	(334)	728	(668)		(1,002)	(1,336)	2,490	661
Provision for income taxes		150	150	(138)	(138)	(138)	300	(275)		(413)	(550)	1,025	272
Net income		$ 214	$ 214	$ (196)	$ (196)	$ (196)	$ 428	$ (393)		$ (589)	$ (786)	$ 1,465	$ 389
Basic and diluted earnings per share		$ 0.01	$ 0.01	$ (0.01)	$ (0.01)	$ (0.01)	$ 0.02	$ (0.01)		$ (0.02)	$ (0.03)	$ 0.05	$ 0.01
Deferred income taxes			$ 150			$ (138)	$ 300	$ (275)		$ (413)	$ (550)	$ 1,025	$ 272
Accounts payable and other liabilities			(364)			334	(728)	668		1,002	1,336	(2,490)	(661)
Net cash provided by operating activities			$ 0			$ 0	$ 0	$ 0		$ 0	$ 0	$ 0	$ 0