SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Oct. 01, 2021	Sep. 30, 2022	Oct. 01, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 121,399	$ 98,297	$ 315,882	$ 261,537
Domestic
Disaggregation of Revenue [Line Items]
Revenue			315,882	261,537
Energy
Disaggregation of Revenue [Line Items]
Revenue	102,625	81,524	262,186	209,977
Energy | Domestic
Disaggregation of Revenue [Line Items]
Revenue			262,186	209,977
Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	18,774	16,773	53,696	51,560
Engineering and Consulting | Domestic
Disaggregation of Revenue [Line Items]
Revenue			53,696	51,560
Commercial
Disaggregation of Revenue [Line Items]
Revenue	8,224	9,309	25,968	24,723
Commercial | Energy
Disaggregation of Revenue [Line Items]
Revenue	6,848	7,741	21,638	20,685
Commercial | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,376	1,568	4,330	4,037
Government
Disaggregation of Revenue [Line Items]
Revenue	65,421	33,718	145,432	93,157
Government | Energy
Disaggregation of Revenue [Line Items]
Revenue	48,073	18,537	96,293	45,766
Government | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	17,348	15,181	49,139	47,391
Utilities
Disaggregation of Revenue [Line Items]
Revenue	47,754	55,269	144,482	143,657
Utilities | Energy
Disaggregation of Revenue [Line Items]
Revenue	47,704	55,245	144,255	143,525
Utilities | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	50	24	227	132
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	21,348	22,264	64,094	65,504
Time-and-materials | Energy
Disaggregation of Revenue [Line Items]
Revenue	7,468	9,104	23,873	25,060
Time-and-materials | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	13,880	13,160	40,221	40,444
Unit-based
Disaggregation of Revenue [Line Items]
Revenue	46,830	54,120	139,070	140,505
Unit-based | Energy
Disaggregation of Revenue [Line Items]
Revenue	43,039	51,470	128,540	132,688
Unit-based | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	3,791	2,650	10,530	7,817
Fixed price
Disaggregation of Revenue [Line Items]
Revenue	53,221	21,913	112,718	55,528
Fixed price | Energy
Disaggregation of Revenue [Line Items]
Revenue	52,118	20,950	109,773	52,229
Fixed price | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	$ 1,103	$ 963	$ 2,945	$ 3,299