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Equity-Based Compensation - RSUs to be Settled in Cash (Details) - Restricted Stock Units to be Settled in Cash - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period for RSUs to be settled in cash     3 years    
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 $ 0.8 $ 1.2 $ 4.5 $ 4.0  
Payments to settle liabilities under compensation plan 0.4 $ 0.1 5.4 $ 6.1  
Accrued liabilities under compensation plan $ 2.6   $ 2.6   $ 3.4