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Employee Benefit Plans - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Multiemployer defined contribution and other benefit plan contributions other than MEP DBP $ 126,191 $ 110,082 $ 85,235
Percentage of contribution by employer of each employee's contribution up to 3% 100.00%    
Percentage of contribution by employer of each employee who contributes between 3% and 6% 50.00%    
Contributions to Quanta 401(k) Plan $ 33,400 26,300 21,900
Contributions to the deferred compensation plans 1,100 1,100 1,000
Deferred compensation obligations included in other long-term liabilities 33,400 30,100  
Investments in company-owned life insurance policies $ 33,500 28,700  
Minimum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Percentage of employee contribution, lower range 3.00%    
Maximum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Percentage of employee contribution, lower range 6.00%    
Multiemployer Defined Contribution and Other Benefit Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Multiemployer defined contribution and other benefit plan contributions other than MEP DBP $ 174,700 $ 171,400 $ 139,300