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SEGMENT INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 27, 2019	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 27, 2019	Dec. 28, 2018	Dec. 29, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 129,416	$ 117,494	$ 104,396	$ 91,793	$ 86,438	$ 71,386	$ 59,833	$ 54,595	$ 443,099	$ 272,252	$ 273,352
Domestic
Disaggregation of Revenue [Line Items]
Revenue									443,099	272,252
Energy
Disaggregation of Revenue [Line Items]
Revenue									370,715	196,833
Energy | Domestic
Disaggregation of Revenue [Line Items]
Revenue									370,715	196,833
Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									72,384	75,419
Engineering and Consulting | Domestic
Disaggregation of Revenue [Line Items]
Revenue									72,384	75,419
Commercial
Disaggregation of Revenue [Line Items]
Revenue									44,206	25,597
Commercial | Energy
Disaggregation of Revenue [Line Items]
Revenue									39,311	20,715
Commercial | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									4,895	4,882
Government
Disaggregation of Revenue [Line Items]
Revenue									124,069	132,988
Government | Energy
Disaggregation of Revenue [Line Items]
Revenue									57,020	62,897
Government | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									67,049	70,091
Utilities
Disaggregation of Revenue [Line Items]
Revenue									274,824	113,667
Utilities | Energy
Disaggregation of Revenue [Line Items]
Revenue									274,384	113,221
Utilities | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									440	446
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue									73,185	73,534
Time-and-materials | Energy
Disaggregation of Revenue [Line Items]
Revenue									18,625	13,790
Time-and-materials | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									54,560	59,744
Unit-based
Disaggregation of Revenue [Line Items]
Revenue									287,369	127,049
Unit-based | Energy
Disaggregation of Revenue [Line Items]
Revenue									272,978	113,749
Unit-based | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									14,391	13,300
Fixed price
Disaggregation of Revenue [Line Items]
Revenue									82,545	71,669
Fixed price | Energy
Disaggregation of Revenue [Line Items]
Revenue									79,112	69,294
Fixed price | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue									$ 3,433	$ 2,375

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+ References

No definition available.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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