Acquisition of Construction Services Businesses - Summary of Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Nov. 30, 2017	Nov. 01, 2017
Business Acquisition [Line Items]
Goodwill	$ 176,485	$ 179,314
New England Utility Constructors Inc [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 800
Contracts receivable				18,300
Other receivables				5,400
Property, plant and equipment				15,100
Prepaid expenses and deposits				1,700
Intangible assets				44,800
Goodwill				32,000
Total assets acquired				118,100
Current liabilities				(18,500)
Other long-term liabilities				(300)
Net assets acquired			$ 99,000	99,300
New England Utility Constructors Inc [Member] | Measurement Period Adjustments [Member]
Business Acquisition [Line Items]
Prepaid expenses and deposits				(100)
Goodwill				200
Total assets acquired				100
Current liabilities				(100)
New England Utility Constructors Inc [Member] | Revised Acquisition Date [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				800
Contracts receivable				18,300
Other receivables				5,400
Property, plant and equipment				15,100
Prepaid expenses and deposits				1,600
Intangible assets				44,800
Goodwill				32,200
Total assets acquired				118,200
Current liabilities				(18,600)
Other long-term liabilities				(300)
Net assets acquired				$ 99,300