Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 13, 2018 USD ($)	Jul. 30, 2014	Sep. 26, 2018 USD ($) restaurant	Sep. 27, 2017 USD ($)	Sep. 26, 2018 USD ($) restaurant supplier segment	Sep. 27, 2017 USD ($)	Dec. 27, 2017 USD ($) supplier	Oct. 10, 2018	Dec. 28, 2016 USD ($)
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | segment					1
Restricted dividend payments, description					Under the 2018 Revolver, Holdings may not make certain payments such as cash dividends, except that it may, inter alia, (i) pay up to $1.0 million per year to repurchase or redeem qualified equity interests of Holdings held by past or present officers, directors, or employees (or their estates) of the Company upon death, disability, or termination of employment, (ii) pay under its income tax receivable agreement (the “TRA”), and, (iii) so long as no default or event of default has occurred and is continuing, (a) make non-cash repurchases of equity interests in connection with the exercise of stock options by directors, officers and management, provided that those equity interests represent a portion of the consideration of the exercise price of those stock options, (b) pay up to $0.5 million in any 12 month consecutive period to redeem, repurchase or otherwise acquire equity interests of any subsidiary that is not a wholly-owned subsidiary from any holder of equity interest in such subsidiary, (c) pay up to $2.5 million per year pursuant to stock option plans, employment agreements, or incentive plans, (d) make up to $5.0 million in other restricted payments per year, and (e) make other restricted payments, subject to its compliance, on a pro forma basis, with (x) a lease-adjusted consolidated leverage ratio not to exceed 4.25 times and (y) the financial covenants applicable to the 2018 Revolver.
Total amount of outstanding debt			$ 71,200,000		$ 71,200,000
Cash available			8,142,000	$ 7,062,000	8,142,000	$ 7,062,000	$ 8,550,000		$ 2,168,000
Recovery of securities lawsuits related legal expenses			(2,036,000)	(634,000)	(6,099,000)	(1,145,000)
Operating lease obligations			279,700,000		279,700,000
Property and equipment owned, net			105,225,000		105,225,000		102,794,000
Impairment of property and equipment			100,000	15,000,000	$ 1,791,000	15,480,000
Number of restaurants closed					6
Closed-store reserve expense (recovery)			(200,000)	1,000,000	$ 4,838,000	1,813,000
Unrecognized tax benefits, accrual of interest or penalties			0		0		0
Unrecognized tax benefits, interest or penalties expenses			0	0	$ 0	$ 0
Federal statutory tax rate					21.00%	35.00%
Percentage of cash savings in taxes realized as a result of utilizing net operating losses payable to pre-IPO stockholders		85.00%
Income tax receivable agreement (income) expense			(571,000)	(19,000)	$ (777,000)	$ 107,000
Deferred tax assets			5,530,000		5,530,000		7,167,000
Total assets			444,905,000		444,905,000		442,711,000
Other accrued expenses and current liabilities			16,048,000		16,048,000		13,270,000
Total current liabilities			51,872,000		51,872,000		47,325,000
Other noncurrent liabilities			20,876,000		20,876,000		12,772,000
Total liabilities			155,856,000		155,856,000		167,761,000
Accumulated deficit			(87,505,000)		(87,505,000)		(98,427,000)
Total stockholders' equity			289,049,000		289,049,000		274,950,000
Total liabilities and stockholder’s equity			444,905,000		444,905,000		$ 442,711,000
Franchise and advertising fee revenue			112,178,000	101,155,000	329,567,000	306,499,000
Franchise expenses			6,342,000	709,000	18,424,000	2,532,000
Total expenses			102,686,000	106,767,000	308,038,000	290,010,000
Income (loss) from operations			9,492,000	(5,612,000)	21,529,000	16,489,000
Income (loss) before provision for income taxes			9,223,000	(6,496,000)	19,618,000	13,911,000
Net income (loss)			6,835,000	(4,039,000)	14,416,000	8,657,000
Calculated under Revenue Guidance in Effect before Topic 606
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Deferred tax assets			4,245,000		4,245,000
Total assets			443,620,000		443,620,000
Other accrued expenses and current liabilities			15,694,000		15,694,000
Total current liabilities			51,518,000		51,518,000
Other noncurrent liabilities			16,567,000		16,567,000
Total liabilities			151,193,000		151,193,000
Accumulated deficit			(84,127,000)		(84,127,000)
Total stockholders' equity			292,427,000		292,427,000
Total liabilities and stockholder’s equity			443,620,000		443,620,000
Franchise and advertising fee revenue			106,737,000		313,644,000
Franchise expenses			853,000		2,385,000
Total expenses			97,197,000		291,999,000
Income (loss) from operations			9,540,000		21,645,000
Income (loss) before provision for income taxes			9,271,000		19,734,000
Net income (loss)			6,883,000		$ 14,532,000
Franchise license
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise agreement renewal term (in years)					20 years
Accounting Standards Update 2014-09
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Deferred tax assets			1,285,000		$ 1,285,000
Total assets			1,285,000		1,285,000
Other accrued expenses and current liabilities			354,000		354,000
Total current liabilities			354,000		354,000
Other noncurrent liabilities			4,309,000		4,309,000
Total liabilities			4,663,000		4,663,000
Accumulated deficit			(3,378,000)		(3,378,000)
Total stockholders' equity			(3,378,000)		(3,378,000)
Total liabilities and stockholder’s equity			1,285,000		1,285,000
Franchise and advertising fee revenue			5,441,000		15,923,000
Franchise expenses			5,489,000		16,039,000
Total expenses			5,489,000		16,039,000
Income (loss) from operations			(48,000)		(116,000)
Income (loss) before provision for income taxes			(48,000)		(116,000)
Net income (loss)			(48,000)		(116,000)
Franchise revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			6,665,000	6,173,000	19,324,000	19,183,000
Franchise revenue | Calculated under Revenue Guidance in Effect before Topic 606
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			6,713,000		19,440,000
Franchise revenue | Accounting Standards Update 2014-09
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			(48,000)		(116,000)
Franchise advertising fee revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			5,489,000	0	16,039,000	0
Franchise advertising fee revenue | Calculated under Revenue Guidance in Effect before Topic 606
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			0		0
Franchise advertising fee revenue | Accounting Standards Update 2014-09
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Franchise and advertising fee revenue			5,489,000		16,039,000
Indefinite-lived Intangible Assets
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Asset impairment charges			$ 0	$ 0	$ 0	$ 0
Supplier Concentration Risk
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of suppliers | supplier					0		1
Supplier Concentration Risk | Accounts Payable
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration					10.00%
Supplier Concentration Risk | Supplier One | Accounts Payable
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration							14.00%
Supplier Concentration Risk | Largest Supplier One | Purchased
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			29.40%	27.00%	28.80%	30.00%
Geographic Concentration Risk | Revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			100.00%		100.00%
Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net			$ 0	$ 280,000	$ 0	$ 280,000
Impairment of property and equipment			84,000	15,035,000	1,791,000	15,480,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net			0	0	0	0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net			0	0	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net			$ 0	$ 280,000	0	$ 280,000
Credit Agreement Two Thousand Eighteen | Senior Secured Revolving Credit Facility
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior secured revolving facility	$ 150,000,000
Senior secured revolving facility term	5 years
Restrictive covenants, maximum annual repurchase or redemption of qualified entity interests, up to					1,000,000.0
Restrictive covenants, maximum annual repurchase, redemption or acquired equity interests, up to					500,000.0
Restrictive covenants, maximum annual payment for stock option plans, employment agreements and incentive plans, up to					2,500,000.0
Restrictive covenants, maximum annual payment for other restricted payments, up to					$ 5,000,000.0
Maximum leverage ratio					4.25
Los Angeles | Geographic Concentration Risk | Revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			69.30%	73.00%	69.00%	75.00%
TEXAS
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants closed				3	4	4
TEXAS | Property, Plant and Equipment
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants that are impaired				8		9
ARIZONA
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants closed			1		1	1
ARIZONA | Property, Plant and Equipment
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants that are impaired			1
CALIFORNIA
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants closed			1		1
CALIFORNIA | Property, Plant and Equipment
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants that are impaired				2		2
Company Operated Properties | Subsequent Event
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants								1
Entity Operated Units
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			212		212
Franchised Units
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			271		271