Acquisitions - Additional information (Details) - USD ($)			3 Months Ended		9 Months Ended
	Jul. 15, 2019	Jul. 02, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2018
Business Acquisition [Line Items]
Income before income taxes			$ 14,215,000	$ 10,844,000	$ 32,206,000	$ 28,378,000
Amortization of intangible assets			1,419,000	$ 743,000	2,888,000	$ 979,000
CSI Electrical Contractors, Inc [Member]
Business Acquisition [Line Items]
Total consideration paid	$ 79,720,000
Preliminary estimated net asset adjustments	633,000
Acquisition related costs			300,000		300,000
Revenues			65,000,000		65,000,000
Income before income taxes			1,700,000		1,700,000
Amortization of intangible assets			1,100,000		1,100,000
Total consideration, net of net asset adjustments	$ 80,353,000
CSI Electrical Contractors, Inc [Member] | Revolving Credit Facility [Member] | Pro Forma [Member]
Business Acquisition [Line Items]
Borrowings under line of credit							$ 79,700,000
CSI Electrical Contractors, Inc [Member] | Margin Guarantee [Member]
Business Acquisition [Line Items]
Change in contingent consideration			1,100,000		1,100,000
CSI Electrical Contractors, Inc [Member] | Certain Performance Targets and Continued Employment of Key Executives [Member]
Business Acquisition [Line Items]
Change in contingent consideration			0		0
Huen Electric Inc [Member]
Business Acquisition [Line Items]
Total consideration paid		$ 47,082,000			47,082,000
Preliminary estimated net asset adjustments		10,749,000			10,834,000
Total consideration, net of net asset adjustments		$ 57,831,000			57,916,000
Huen Electric Inc [Member] | Margin Guarantee [Member]
Business Acquisition [Line Items]
Change in contingent consideration			0		1,400,000
Huen Electric Inc [Member] | Certain Performance Targets and Continued Employment of Key Executives [Member]
Business Acquisition [Line Items]
Change in contingent consideration			$ 900,000		$ 1,400,000