SEGMENT AND GEOGRAPHICAL INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 03, 2020	Jun. 28, 2019	Jul. 03, 2020	Jun. 28, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 83,549	$ 104,396	$ 189,575	$ 196,189
Domestic
Disaggregation of Revenue [Line Items]
Revenue	83,549	104,396	189,575	196,189
Energy
Disaggregation of Revenue [Line Items]
Revenue	66,708	85,283	154,506	159,975
Energy | Domestic
Disaggregation of Revenue [Line Items]
Revenue	66,708	85,283	154,506	159,975
Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	16,841	19,113	35,069	36,214
Engineering and Consulting | Domestic
Disaggregation of Revenue [Line Items]
Revenue	16,841	19,113	35,069	36,214
Commercial
Disaggregation of Revenue [Line Items]
Revenue	10,193	8,168	20,296	18,661
Commercial | Energy
Disaggregation of Revenue [Line Items]
Revenue	8,889	6,840	17,618	16,035
Commercial | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,304	1,328	2,678	2,626
Government
Disaggregation of Revenue [Line Items]
Revenue	36,640	32,242	75,162	56,775
Government | Energy
Disaggregation of Revenue [Line Items]
Revenue	21,701	14,583	43,428	23,445
Government | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	14,939	17,659	31,734	33,330
Utilities
Disaggregation of Revenue [Line Items]
Revenue	36,716	63,986	94,117	120,753
Utilities | Energy
Disaggregation of Revenue [Line Items]
Revenue	36,118	63,860	93,460	120,495
Utilities | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	598	126	657	258
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	25,814	17,689	53,917	35,002
Time-and-materials | Energy
Disaggregation of Revenue [Line Items]
Revenue	12,125	3,093	26,136	7,348
Time-and-materials | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	13,689	14,596	27,781	27,654
Unit-based
Disaggregation of Revenue [Line Items]
Revenue	30,893	67,349	84,887	127,792
Unit-based | Energy
Disaggregation of Revenue [Line Items]
Revenue	28,900	63,757	79,789	120,629
Unit-based | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	1,993	3,592	5,098	7,163
Fixed price
Disaggregation of Revenue [Line Items]
Revenue	26,842	19,358	50,771	33,395
Fixed price | Energy
Disaggregation of Revenue [Line Items]
Revenue	25,683	18,433	48,581	31,998
Fixed price | Engineering and Consulting
Disaggregation of Revenue [Line Items]
Revenue	$ 1,159	$ 925	$ 2,190	$ 1,397