Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)			3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 24, 2020 USD ($) restaurant	Jul. 30, 2014	Sep. 23, 2020 USD ($) restaurant	Sep. 25, 2019 USD ($) restaurant	Sep. 23, 2020 USD ($) restaurant item segment	Sep. 25, 2019 USD ($) restaurant	Dec. 25, 2019 USD ($) item	Oct. 30, 2020 USD ($)	Jul. 31, 2020 restaurant
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | segment					1
Historical dine-in percentage					20.00%
Number of Restaurants Permanently Closed | restaurant			0		0
Repayments of Lines of Credit					$ 65,700,000	$ 15,000,000
Revolving line of credit			$ 83,800,000		83,800,000		$ 97,000,000
Total amount of outstanding debt			83,800,000		83,800,000
Cash available			29,529,000		29,529,000		8,070,000
Gain on recovery of insurance proceeds			2,000,000		2,000,000
Insurance proceeds					100,000	10,000,000.0
Proceeds from disposition of restaurants						4,770,000
Loss on disposition of restaurants				$ (11,000)		(5,062,000)
Property and equipment owned, net			80,545,000		80,545,000		91,778,000
Impairment of certain property and equipment owned			1,506,000	56,000	2,935,000	283,000
Impairment of Right-of-Use Assets					543,000
Asset Impairment Charges			1,500,000	100,000	3,500,000	300,000
Unrecognized tax benefits, accrual of interest or penalties			0		0		$ 0
Interest related to NOPA			100,000		100,000
Unrecognized tax benefits, interest or penalties expenses				0		0
Percentage of cash savings in taxes realized as a result of utilizing net operating losses payable to pre-IPO stockholders		85.00%
Tax Receivable Agreement (Income) Expenses			(144,000)	$ (197,000)	26,000	$ (120,000)
AMT Credits, CARES Act			$ 400,000		400,000
Maximum
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Tax Receivable Agreement (Income) Expenses					$ 100,000
Supplier Concentration Risk
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number Of Suppliers | item					0		1
Supplier Concentration Risk | Supplier One | Accounts Payable
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration							11.70%
Supplier Concentration Risk | Largest Supplier One | Purchased
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			26.70%	29.00%	26.80%	28.90%
Geographic Concentration Risk | Revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			100.00%	100.00%	100.00%	100.00%
Fair Value, Measurements, Recurring [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative Liability, Fair Value, Gross Liability			$ 1,328,000		$ 1,328,000
Derivative Liability, Fair Value, Gross Asset							$ 360,000
Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net				$ 0		$ 0
Right-of-Use Assets, Net			911,000		911,000
Level 1 | Fair Value, Measurements, Recurring [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative Liability, Fair Value, Gross Liability			0		0
Derivative Liability, Fair Value, Gross Asset							0
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
Right-of-Use Assets, Net			0		0
Level 2 | Fair Value, Measurements, Recurring [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative Liability, Fair Value, Gross Liability			1,328,000		1,328,000
Derivative Liability, Fair Value, Gross Asset							360,000
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
Right-of-Use Assets, Net			0		0
Level 3 | Fair Value, Measurements, Recurring [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative Liability, Fair Value, Gross Liability			0		0
Derivative Liability, Fair Value, Gross Asset							$ 0
Level 3 | Fair Value, Measurements, Nonrecurring
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Property and equipment owned, net				$ 0		0
Right-of-Use Assets, Net			911,000		911,000
2018 Credit Agreement | Revolving Credit Facility
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Repayments of Lines of Credit			55,000,000.0		13,200,000	$ 15,000,000.0
Revolving line of credit			83,800,000		83,800,000
Amount of borrowings available			$ 57,800,000		$ 57,800,000
2018 Credit Agreement | Revolving Credit Facility | Subsequent Event
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Repayments of Lines of Credit	$ 28,000,000.0
Revolving line of credit								$ 55,800,000
Amount of borrowings available								$ 85,800,000
Texas
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			1		1
Number of restaurants closed | restaurant				1		1
California
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			4	1	4	1
Number of restaurants closed | restaurant				1		1
Greater Los Angeles area market | Geographic Concentration Risk | Revenue
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of concentration			71.20%	71.50%	71.50%	70.00%
Closed Store
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Closed-store reserve expense			$ 300,000	$ 300,000	$ 1,100,000	$ 1,000,000.0
Company-operated
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			196		196
Number of Restaurants Temporary Closed | restaurant			40		65
Number of Restaurants Remain Closed | restaurant			3		3				2
Proceeds from disposition of restaurants						4,800,000
Loss on disposition of restaurants						$ (5,100,000)
Company-operated | Subsequent Event
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of Restaurants Temporary Closed | restaurant	8
Company-operated | Subsequent Event | Minimum
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Temporary closure period	1 day
Company-operated | Subsequent Event | Maximum
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Temporary closure period	3 days
Company-operated | San Francisco
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of Restaurants Sold | restaurant				4		4
Company-operated | Phoenix
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of Restaurants Sold | restaurant				7		7
Franchised
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of restaurants | restaurant			282		282
Number of Restaurants Temporary Closed | restaurant			15		36
Number of Restaurants Remain Closed | restaurant				3		3			3