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SEGMENT INFORMATION - Concentration risk (Details) | 12 Months Ended
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Dec. 27, 2019 customer state | Dec. 28, 2018 customer | Dec. 29, 2017 customer
Concentration Risk [Line Items] |  |  |
Number of States in which Entity Operates | state | 24 |  |
Revenue from Contract with Customer Benchmark | CALIFORNIA |  |  |
Concentration Risk [Line Items] |  |  |
Customer concentration risk percentage | 41.10% | 35.00% | 30.00%
Revenue from Contract with Customer Benchmark | NEW YORK |  |  |
Concentration Risk [Line Items] |  |  |
Customer concentration risk percentage | 27.20% | 29.00% | 46.00%
Customer Concentration Risk | Revenue from Contract with Customer Benchmark |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers | 2 | 1 | 2
Customer concentration risk percentage | 29.10% | 19.00% | 38.00%
Customer Concentration Risk | Revenue from Contract with Customer Benchmark | Energy Segment |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers | 2 | 1 | 1
Customer concentration risk percentage | 34.70% | 18.50% | 22.30%
Customer Concentration Risk | Revenue from Contract with Customer Benchmark | Engineering And Consulting Segment |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers | 1 | 3 | 7
Customer concentration risk percentage | 25.00% | 10.00% | 10.00%
Top Ten Customers | Customer Concentration Risk | Revenue from Contract with Customer Benchmark |  |  |
Concentration Risk [Line Items] |  |  |
Number of customers | 10 | 10 | 10
Customer concentration risk percentage | 50.60% | 56.90% | 64.40%

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