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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
Mar. 31, 2018
Mar. 31, 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 $ 1.3 $ 2.7  
Payments to settle liabilities under compensation plan 2.2 $ 2.4  
Accrued liabilities under compensation plan $ 3.7   $ 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