Quarterly report pursuant to Section 13 or 15(d)

Equity-Based Compensation - RSUs to be Settled in Cash (Detail)

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Equity-Based Compensation - RSUs to be Settled in Cash (Detail) - Restricted Stock Units to be Settled in Cash - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Amount in cash received for each RSU is equal to the fair value of this number of Quanta common stock shares (in shares).     1    
Compensation expense related to RSUs to be settled in cash $ 1.5 $ 1.2 $ 2.8 $ 3.9  
Payments to settle liabilities under compensation plan 3.8 $ 3.8 6.0 $ 6.1  
Accrued liabilities under compensation plan $ 1.4   $ 1.4   $ 4.6
Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period for RSUs to be settled in cash     2 years    
Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period for RSUs to be settled in cash     3 years