Acquisitions (2016 Acquisitions) - Additional Information (Detail) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($) Acquisition shares	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Cash paid or payable	$ 75,941	$ 110,578
Contingent consideration	18,683	1,001
Value of Quanta common stock and exchangeable shares issued	$ 1,508	$ 10,127
Acquisitions 2016 [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	5
Cash paid or payable	$ 75,900
Contingent consideration	$ 18,700
Number of shares granted for acquired companies \| shares	70,840
Value of Quanta common stock and exchangeable shares issued	$ 1,500
Acquisitions 2016 [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Contingent consideration	$ 39,500
Acquisitions 2016 [Member] \| Electric Power Infrastructure Services Business [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	4
Acquisitions 2016 [Member] \| Electric Power Infrastructure Services Business [Member] \| Australia [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	1
Acquisitions 2016 [Member] \| Electric Power Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	1
Acquisitions 2016 [Member] \| Electric Power Infrastructure Services Business [Member] \| Canada [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	2
Acquisitions 2016 [Member] \| Oil and Gas Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions \| Acquisition	1

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