Acquisitions - Summary of acquisition by acquisition (Details) - USD ($) $ in Thousands			9 Months Ended	15 Months Ended
	Jul. 15, 2019	Jul. 02, 2018	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill			$ 65,557	$ 65,557	$ 56,588
CSI Electrical Contractors, Inc [Member]
Business Acquisition [Line Items]
Consideration paid	$ 79,720
Preliminary estimated net asset adjustments	633
Total consideration, net of net asset adjustments	80,353
Accounts receivable, net	59,579
Contract assets	38,970
Other current assets	83
Property and equipment	7,964
Operating lease right-of-use assets	9,933
Intangible assets	26,000
Other long term assets	149
Accounts payable	(29,533)
Accrued salaries and benefits	(8,091)
Contract liabilities	(18,934)
Current portion of operating lease obligations	(2,526)
Other current liabilities	(4,776)
Operating lease obligations, net of current maturities	(7,407)
Long-term debt	(20)
Net identifiable assets and liabilities	71,391
Goodwill	$ 8,962
Huen Electric Inc [Member]
Business Acquisition [Line Items]
Consideration paid		$ 47,082	47,082
Preliminary estimated net asset adjustments		10,749	10,834
Total consideration, net of net asset adjustments		57,831	57,916
Accounts receivable, net		33,903	33,696	33,696
Contract assets		10,570	11,580	11,580
Other current and long term assets		88	77	77
Property and equipment		3,188	3,188	3,188
Intangible assets		0	24,300	24,300
Accounts payable		(9,592)	(10,866)	(10,866)
Contract liabilities		(6,394)	(5,869)	(5,869)
Other current liabilities		(6,570)	(6,521)	(6,521)
Net identifiable assets and liabilities		25,193	49,585	49,585
Unallocated intangible assets		9,800	0	0
Total acquired assets and liabilities		34,993	49,585	49,585
Fair value of acquired noncontrolling interest		(1,273)	(1,280)	(1,280)
Goodwill		$ 24,111	$ 9,611	9,611
Consideration paid				0
Preliminary estimated net asset adjustments				85
Total consideration, net of net asset adjustments				85
Accounts receivable, net				(207)
Contract assets				1,010
Other current and long term assets				(11)
Property and equipment				0
Intangible assets				24,300
Accounts payable				(1,274)
Contract liabilities				525
Other current liabilities				49
Net identifiable assets and liabilities				24,392
Unallocated intangible assets				(9,800)
Total acquired assets and liabilities				14,592
Fair value of acquired noncontrolling interest				(7)
Goodwill				$ (14,500)