Acquisitions (2015 Acquisitions) - Additional Information (Detail) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) Entity shares
Business Acquisition [Line Items]
Cash paid or payable \| $	$ 75,941	$ 110,578
Value of Quanta common stock and exchangeable shares issued \| $	1,508	10,127
Contingent consideration \| $	$ 18,683	1,001
Acquisitions 2015 [Member]
Business Acquisition [Line Items]
Cash paid or payable \| $		$ 110,600
Number of shares granted for acquired companies \| shares		461,037
Value of Quanta common stock and exchangeable shares issued \| $		$ 10,100
Contingent consideration \| $		$ 1,000
Number of business acquisitions		11
Acquisitions 2015 [Member] \| Electric Power Infrastructure Services Business [Member]
Business Acquisition [Line Items]
Number of business acquisitions		8
Acquisitions 2015 [Member] \| Electric Power Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions		4
Acquisitions 2015 [Member] \| Electric Power Infrastructure Services Business [Member] \| Australia [Member]
Business Acquisition [Line Items]
Number of business acquisitions		1
Acquisitions 2015 [Member] \| Electric Power Infrastructure Services Business [Member] \| Canada [Member]
Business Acquisition [Line Items]
Number of business acquisitions		3
Acquisitions 2015 [Member] \| Oil and Gas Infrastructure Services Business [Member]
Business Acquisition [Line Items]
Number of business acquisitions		3
Acquisitions 2015 [Member] \| Oil and Gas Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions		1
Acquisitions 2015 [Member] \| Oil and Gas Infrastructure Services Business [Member] \| Australia [Member]
Business Acquisition [Line Items]
Number of business acquisitions		1
Acquisitions 2015 [Member] \| Oil and Gas Infrastructure Services Business [Member] \| Canada [Member]
Business Acquisition [Line Items]
Number of business acquisitions		1

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