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Stock-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, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
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 $ 2.6 $ 0.8 $ 5.3 $ 4.5  
Payments to settle liabilities under compensation plan 0.8 $ 0.4 4.3 $ 5.4  
Accrued liabilities under compensation plan $ 4.5   $ 4.5   $ 4.3