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SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 27, 2024	Sep. 27, 2024	Jun. 28, 2024	Mar. 29, 2024	Dec. 29, 2023	Sep. 29, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 27, 2024	Dec. 29, 2023	Dec. 30, 2022
Revenue
Revenue	$ 144,061	$ 158,252	$ 140,996	$ 122,489	$ 155,677	$ 132,738	$ 119,077	$ 102,603	$ 565,798	$ 510,095	$ 429,138
Domestic
Revenue
Revenue									565,798	510,095	429,138
Energy
Revenue
Revenue									473,309	426,976	357,460
Energy | Domestic
Revenue
Revenue									473,309	426,976	357,460
Engineering and Consulting
Revenue
Revenue									92,489	83,119	71,678
Engineering and Consulting | Domestic
Revenue
Revenue									92,489	83,119	71,678
Commercial
Revenue
Revenue									41,620	37,028	35,348
Commercial | Energy
Revenue
Revenue									34,072	31,162	29,782
Commercial | Engineering and Consulting
Revenue
Revenue									7,548	5,866	5,566
Government
Revenue
Revenue									266,774	236,907	192,463
Government | Energy
Revenue
Revenue									182,079	159,935	126,494
Government | Engineering and Consulting
Revenue
Revenue									84,695	76,972	65,969
Utilities
Revenue
Revenue									257,404	236,160	201,327
Utilities | Energy
Revenue
Revenue									257,158	235,879	201,184
Utilities | Engineering and Consulting
Revenue
Revenue									246	281	143
Time-and-materials
Revenue
Revenue									102,312	99,112	86,075
Time-and-materials | Energy
Revenue
Revenue									34,381	35,582	32,491
Time-and-materials | Engineering and Consulting
Revenue
Revenue									67,931	63,530	53,584
Unit-based
Revenue
Revenue									224,793	214,793	194,805
Unit-based | Energy
Revenue
Revenue									205,117	199,040	180,509
Unit-based | Engineering and Consulting
Revenue
Revenue									19,676	15,753	14,296
Fixed price
Revenue
Revenue									238,693	196,190	148,258
Fixed price | Energy
Revenue
Revenue									233,811	192,354	144,460
Fixed price | Engineering and Consulting
Revenue
Revenue									$ 4,882	$ 3,836	$ 3,798

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