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Acquisitions - Additional information (Details) - USD ($) $ in Thousands			12 Months Ended			18 Months Ended
	Jul. 15, 2019	Jul. 02, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Jan. 01, 2018
Business Acquisition [Line Items]
Revenues				$ 1,531,169
Income before income taxes			$ 50,442	43,068	$ 24,640
Intangible asset amortization			3,849	$ 1,843	$ 499
CSI Electrical Contractors, Inc [Member]
Business Acquisition [Line Items]
Preliminary estimated net asset adjustments	$ 633		987
Total consideration, net of net asset adjustments	$ 80,353		80,707
Acquisition related costs			600			$ 600
Revenues			137,700
Income before income taxes			1,500
Intangible asset amortization			1,800
CSI Electrical Contractors, Inc [Member] | Revolving Credit Facility [Member] | Pro Forma [Member]
Business Acquisition [Line Items]
Borrowings under line of credit							$ 79,700
CSI Electrical Contractors, Inc [Member] | Margin Guarantee [Member]
Business Acquisition [Line Items]
Change in contingent consideration			2,000
CSI Electrical Contractors, Inc [Member] | Certain Performance Targets and Continued Employment of Key Executives [Member]
Business Acquisition [Line Items]
Change in contingent consideration			400
Huen Electric Inc [Member]
Business Acquisition [Line Items]
Preliminary estimated net asset adjustments		$ 10,749				10,834
Total consideration, net of net asset adjustments		$ 57,831				$ 57,916
Huen Electric Inc [Member] | Margin Guarantee [Member]
Business Acquisition [Line Items]
Change in contingent consideration			1,500
Huen Electric Inc [Member] | Certain Performance Targets and Continued Employment of Key Executives [Member]
Business Acquisition [Line Items]
Change in contingent consideration			$ 1,900

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount of direct costs of the business combination including legal, accounting, and other costs incurred to consummate the business acquisition.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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