Acquisitions - 2016 Acquisitions (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Acquisition shares	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Contingent consideration, maximum		$ 139,500
Contingent consideration	$ 18,700	$ 51,100	$ 1,000
Acquisitions 2016
Business Acquisition [Line Items]
Number of business acquisitions | Acquisition	5
Cash paid or payable	$ 75,941
Number of shares granted for acquired companies (in shares) | shares	70,840
Value of Quanta common stock issued	$ 1,508
Contingent consideration, maximum	39,500
Contingent consideration	$ 18,683
Acquisitions 2016 | Electric Power Infrastructure Services Business
Business Acquisition [Line Items]
Number of business acquisitions | Acquisition	4