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BUSINESS COMBINATIONS (Acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands							3 Months Ended								12 Months Ended
	Oct. 28, 2019	Jul. 02, 2019	Mar. 08, 2019	Nov. 09, 2018	Oct. 01, 2018	Apr. 30, 2018	Dec. 27, 2019	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 27, 2019	Dec. 28, 2018	Dec. 29, 2017	Jun. 26, 2019
Consideration for acquisitions
Cash paid															$ 71,756	$ 124,344	$ 14,603
Cash paid at closing															27,000
Other receivable for working capital adjustment																(63)	(113)
Allocation of acquired assets
Goodwill							$ 127,647				$ 97,748				127,647	97,748	38,184
Additional amortization expense charge due to increase in fair value of intangible assets															11,600	4,500	2,400
Revenue and Income from operations
Revenues.							129,416	$ 117,494	$ 104,396	$ 91,793	86,438	$ 71,386	$ 59,833	$ 54,595	443,099	272,252	273,352
Income from operations							$ 5,529	1,295	$ 2,773	$ (234)	$ 1,679	$ 4,913	$ 4,205	$ 1,974	9,363	12,771	$ 13,704
Delayed Draw Term Loan Facility
Consideration for acquisitions
Maximum borrowing capacity																		$ 50,000
E3, Inc.
Consideration for acquisitions
Tax deductible goodwill	$ 22,000
Value of shares issued and issuable	$ 5,000
Number of trading days considered for calculation of weighted average price per share prior to closing date	10 days
Period over which financial targets must be met	3 years
Maximum Payout	$ 12,000
Contingent consideration	7,000
Cash paid	25,217
Other working capital adjustment	1,973
Total consideration	39,190
Cash paid at closing	27,000
Allocation of acquired assets
Current assets (1)	5,316
Non-current assets (2)	341
Cash	2,264
Equipment and leasehold improvements, net	409
Right-of-use asset	6,766
Current lease liability	(593)
Non-current lease liability	(7,870)
Liabilities	(3,138)
Goodwill	21,995
Net assets acquired	39,190
Revenue and Income from operations
Revenues.															3,700
Income from operations															1,200
E3, Inc. | Other General And Administrative Expense
Revenue and Income from operations
Acquisition costs															100
E3, Inc. | Maximum
Consideration for acquisitions
Earn-Out Payments	12,000
Total consideration	44,000
E3, Inc. | Backlog
Allocation of acquired assets
Intangible assets	2,500
E3, Inc. | Customer relationships
Allocation of acquired assets
Intangible assets	8,300
E3, Inc. | Tradename
Allocation of acquired assets
Intangible assets	2,000
E3, Inc. | Non-compete agreements
Allocation of acquired assets
Intangible assets	$ 900
Onsite Energy Corporation
Consideration for acquisitions
Tax deductible goodwill		$ 6,200
Cash paid		24,411
Other working capital adjustment		494
Total consideration		24,905
Allocation of acquired assets
Current assets (1)		22,254
Non-current assets (2)		10
Equipment and leasehold improvements, net		39
Right-of-use asset		828
Current lease liability		(168)
Non-current lease liability		(660)
Liabilities		(12,222)
Goodwill		6,150
Net assets acquired		24,905
Decrease in current assets															200
Increase in carrying value of goodwill															900
Revenue and Income from operations
Revenues.															7,700
Income from operations															1,800
Onsite Energy Corporation | General and administrative expenses
Revenue and Income from operations
Acquisition costs															300
Onsite Energy Corporation | Backlog
Allocation of acquired assets
Intangible assets		800
Onsite Energy Corporation | Customer relationships
Allocation of acquired assets
Intangible assets		7,374
Onsite Energy Corporation | Tradename
Allocation of acquired assets
Intangible assets		500
The Weidt Group
Consideration for acquisitions
Tax deductible goodwill			$ 11,500
Cash paid			22,136
Total consideration			22,136
Allocation of acquired assets
Current assets (1)			2,317
Non-current assets (2)			25
Equipment and leasehold improvements, net			198
Right-of-use asset			1,730
Current lease liability			(245)
Non-current lease liability			(1,533)
Liabilities			(612)
Goodwill			11,546
Net assets acquired			22,136
Increase in carrying value of backlog, customer relationships, tradename, and developed technology															900
Decrease in carrying value of liabilities and goodwill															1,200
Additional amortization expense charge due to increase in fair value of intangible assets								100
Revenue and Income from operations
Acquisition costs															400
Revenues.															11,400
Income from operations															1,900
The Weidt Group | Backlog
Allocation of acquired assets
Intangible assets			750
The Weidt Group | Customer relationships
Allocation of acquired assets
Intangible assets			4,240
The Weidt Group | Tradename
Allocation of acquired assets
Intangible assets			550
The Weidt Group | Developed technology
Allocation of acquired assets
Intangible assets			$ 3,170
Lime Energy
Consideration for acquisitions
Cash paid					$ 122,439
Total consideration				$ 120,000	122,439
Cash paid at closing				$ 56,400
Allocation of acquired assets
Current assets (1)					45,401
Non-current assets (2)					10,605
Cash					1,090
Equipment and leasehold improvements, net					1,892
Liabilities					(33,603)
Goodwill					47,744
Net assets acquired					122,439
Increase in carrying value of backlog, customer relationships, tradename, and developed technology															16,200
Increase in carrying value of goodwill															(16,200)
Additional amortization expense charge due to increase in fair value of intangible assets								$ 1,200
Unaudited pro forma financial information
Pro forma revenue															473,655	452,082
Pro forma income from operations															10,937	19,694
Pro forma net income															$ 6,161	$ 11,934
Earnings per share:
Basic (in dollars per share)															$ 0.55	$ 1.09
Diluted (in dollars per share)															$ 0.52	$ 1.04
Weighted average shares outstanding:
Basic (in shares)															11,162	10,990
Diluted (in shares)															11,766	11,489
Revenue and Income from operations
Acquisition costs															$ 200
Revenues.															158,700	$ 181,500
Income from operations															$ 3,500	$ 8,400
Lime Energy | Backlog
Allocation of acquired assets
Intangible assets					1,300
Lime Energy | Customer relationships
Allocation of acquired assets
Intangible assets					35,000
Lime Energy | Tradename
Allocation of acquired assets
Intangible assets					5,970
Lime Energy | Developed technology
Allocation of acquired assets
Intangible assets					$ 7,040
NAM | Maximum
Consideration for acquisitions
Total consideration						$ 4,000
Willdan Energy Solutions | Onsite Energy Corporation | Maximum
Consideration for acquisitions
Total consideration		$ 26,400

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