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SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 29, 2023	Sep. 29, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 30, 2022	Sep. 30, 2022	Jul. 01, 2022	Apr. 01, 2022	Dec. 29, 2023	Dec. 30, 2022	Dec. 31, 2021
Revenue
Revenue	$ 155,677	$ 132,738	$ 119,077	$ 102,603	$ 113,256	$ 121,399	$ 102,645	$ 91,838	$ 510,095	$ 429,138	$ 353,755
Domestic
Revenue
Revenue									510,095	429,138	353,755
Energy
Revenue
Revenue									426,976	357,460	286,384
Energy | Domestic
Revenue
Revenue									426,976	357,460	286,384
Engineering and Consulting
Revenue
Revenue									83,119	71,678	67,371
Engineering and Consulting | Domestic
Revenue
Revenue									83,119	71,678	67,371
Commercial
Revenue
Revenue									37,028	35,348	29,864
Commercial | Energy
Revenue
Revenue									31,162	29,782	24,541
Commercial | Engineering and Consulting
Revenue
Revenue									5,866	5,566	5,323
Government
Revenue
Revenue									236,907	192,463	127,148
Government | Energy
Revenue
Revenue									159,935	126,494	65,249
Government | Engineering and Consulting
Revenue
Revenue									76,972	65,969	61,899
Utilities
Revenue
Revenue									236,160	201,327	196,743
Utilities | Energy
Revenue
Revenue									235,879	201,184	196,594
Utilities | Engineering and Consulting
Revenue
Revenue									281	143	149
Time-and-Materials Contract
Revenue
Revenue									99,112	86,075	86,213
Time-and-Materials Contract | Energy
Revenue
Revenue									35,582	32,491	34,004
Time-and-Materials Contract | Engineering and Consulting
Revenue
Revenue									63,530	53,584	52,209
Unit-based Contract
Revenue
Revenue									214,793	194,805	190,999
Unit-based Contract | Energy
Revenue
Revenue									199,040	180,509	180,311
Unit-based Contract | Engineering and Consulting
Revenue
Revenue									15,753	14,296	10,688
Fixed-Price Contract
Revenue
Revenue									196,190	148,258	76,543
Fixed-Price Contract | Energy
Revenue
Revenue									192,354	144,460	72,069
Fixed-Price Contract | Engineering and Consulting
Revenue
Revenue									$ 3,836	$ 3,798	$ 4,474

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