Revenue Recognition - Reserves for Variable Consideration and Credit Risk (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019	Dec. 28, 2019
Adjustments to the allowances for accounts receivable and unbilled services related to reserves for credit risk
Beginning balance			$ 370	$ 639	$ 639
Cumulative effect of a change in accounting principle related to ASC 326			203		0
Provision for credit loss expense, net of recoveries			228		173
Amounts written off			(128)		(442)
Ending balance	$ 673		673		$ 370
Bad debt expense, net of recoveries of previously written off allowances
Bad debt expense (recovery), net	271	$ 142	228	173
Reimbursable expenses	$ 13,150	$ 13,978	$ 44,395	$ 38,991