SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 03, 2026	Jul. 04, 2025	Jul. 03, 2026	Jul. 04, 2025
Revenue
Revenue	$ 231,028	$ 173,473	$ 386,142	$ 325,859
United States
Revenue
Revenue	231,028	173,473	386,142	325,859
Commercial
Revenue
Revenue	60,820	19,848	82,055	35,174
Government
Revenue
Revenue	96,231	80,763	165,267	153,359
Utilities
Revenue
Revenue	73,977	72,862	138,820	137,326
Time-and-materials
Revenue
Revenue	33,188	30,632	64,088	60,294
Unit-based
Revenue
Revenue	69,946	64,737	126,616	118,699
Fixed price
Revenue
Revenue	127,894	78,104	195,438	146,866
Energy
Revenue
Revenue	202,589	146,749	330,557	272,997
Energy \| United States
Revenue
Revenue	202,589	146,749	330,557	272,997
Energy \| Commercial
Revenue
Revenue	58,849	18,093	78,238	31,608
Energy \| Government
Revenue
Revenue	69,834	55,839	113,680	104,163
Energy \| Utilities
Revenue
Revenue	73,906	72,817	138,639	137,226
Energy \| Time-and-materials
Revenue
Revenue	14,541	11,733	27,241	23,335
Energy \| Unit-based
Revenue
Revenue	61,714	58,250	111,331	105,957
Energy \| Fixed price
Revenue
Revenue	126,334	76,766	191,985	143,705
Engineering and Consulting
Revenue
Revenue	28,439	26,724	55,585	52,862
Engineering and Consulting \| United States
Revenue
Revenue	28,439	26,724	55,585	52,862
Engineering and Consulting \| Commercial
Revenue
Revenue	1,971	1,755	3,817	3,566
Engineering and Consulting \| Government
Revenue
Revenue	26,397	24,924	51,587	49,196
Engineering and Consulting \| Utilities
Revenue
Revenue	71	45	181	100
Engineering and Consulting \| Time-and-materials
Revenue
Revenue	18,647	18,899	36,847	36,959
Engineering and Consulting \| Unit-based
Revenue
Revenue	8,232	6,487	15,285	12,742
Engineering and Consulting \| Fixed price
Revenue
Revenue	$ 1,560	$ 1,338	$ 3,453	$ 3,161