Employee Benefit Plans - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 110,082	$ 85,235	$ 77,015
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	$ 26,300	21,900	17,700
Contributions to the deferred compensation plans	1,100	1,000	1,000
Deferred compensation obligations included in other long-term liabilities	30,100	19,100
Investments in company-owned life insurance policies	$ 28,700	17,900
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%
Union Employees
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	$ 171,400	$ 139,300	$ 147,100