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SEGMENT INFORMATION - Concentration risk (Details)	12 Months Ended
	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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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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The number of states the entity operates in as of the balance sheet date.

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Represents the number of customers on whom the entity significantly rely giving rise to concentration risk.

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