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SUPPLEMENTAL FINANCIAL STATEMENT DATA - Accounts Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2019	Dec. 28, 2018	Dec. 29, 2017	Dec. 27, 2019	Dec. 28, 2018
Accounts receivable
Other assets				$ 14,075	$ 1,626
Total accounts receivable, gross				174,144	115,265
Allowance for doubtful accounts	$ (1,147)	$ (442)	$ (369)	(1,147)	(442)
Total accounts receivable, net				172,997	114,823
Movements in the allowance for doubtful accounts
Balance as of the beginning of the year	442	369	785
(Recovery of) provision for doubtful accounts	1,051	470	(189)
Write-offs of uncollectible accounts	(346)	(397)	(227)
Balance as of the end of the year	1,147	$ 442	$ 369
Cash proceeds from sale of trade receivables	4,900
Discounts on trade receivables sold	$ 200
Accounts Receivable | Credit concentration risk | Dormitory Authority-State of New York
Movements in the allowance for doubtful accounts
Customer concentration risk percentage		21.00%
Accounts Receivable | Credit concentration risk | Los Angeles Department Of Water And Power
Movements in the allowance for doubtful accounts
Customer concentration risk percentage	16.00%
Billed
Accounts receivable
Total accounts receivable, gross				58,651	61,788
Unbilled
Accounts receivable
Total accounts receivable, gross				96,041	45,158
Contract retentions
Accounts receivable
Total accounts receivable, gross				$ 5,377	$ 6,693

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