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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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