Stock-Based Compensation Expense - Stock Incentive Plan (Details) - USD ($)			1 Months Ended	9 Months Ended
	Jan. 01, 2019	Sep. 25, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 26, 2018
Incentive stock options
Stock-Based Compensation
Annual increase in the number of shares to the shares of common stock (in shares)	524,993
2015 Stock Incentive Plan
Stock-Based Compensation
Number of options granted (in shares)		0
Number of shares available for future issuance (in shares)					0
2018 Stock Incentive Plan | Non-employee directors
Stock-Based Compensation
Maximum cash fee paid			$ 500,000
2018 Stock Incentive Plan | Board of directors
Stock-Based Compensation
Maximum cash fee paid			$ 800,000
2018 Stock Incentive Plan | Stock Options
Stock-Based Compensation
Number of options granted (in shares)				1,160,106
Number of shares available for future issuance (in shares)				653,912
Number of shares available for grant (in shares)			1,181,972
Period of options			10 years
Percentage of annual increase to the total number of common stock outstanding (as a percent)			4.00%
2018 Stock Incentive Plan | Incentive stock options
Stock-Based Compensation
Maximum number of shares issued (in shares)			7,500,000