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Acquisitions - 2015 Acquisitions (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Acquisition
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Business Acquisition [Line Items]      
Contingent consideration $ 1.0 $ 51.1 $ 18.7
Acquisitions 2015      
Business Acquisition [Line Items]      
Number of business acquisitions | Acquisition 11    
Cash paid or payable $ 110.6    
Number of shares granted for acquired companies (in shares) | shares 461,037    
Value of Quanta common stock issued $ 10.1    
Contingent consideration $ 1.0    
Acquisitions 2015 | Electric Power Infrastructure Services Business      
Business Acquisition [Line Items]      
Number of business acquisitions | Acquisition 8    
Acquisitions 2015 | Oil and Gas Infrastructure Services Business      
Business Acquisition [Line Items]      
Number of business acquisitions | Acquisition 3