Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percent of employees covered by collective bargaining agreements	34.00%
Contributions	$ 121,978	$ 134,197	$ 126,191
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	$ 45,900	41,400	33,400
Contributions to the deferred compensation plans	1,300	1,100	1,100
Discretionary contributions	0	0	0
Deferred compensation obligations included in other long-term liabilities	58,200	47,300
Investments in company-owned life insurance policies	$ 56,500	45,800
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]
Contributions	$ 188,600	$ 201,300	$ 174,700

Defined Contribution Plan, Employer Matching Contribution, Percent of Match By Employer Up To Specific Percentage

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Defined Contribution Plan, Employer Matching Contribution, Percent of Match By Employer Of Each Employee Who Contributes To Plan

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Amount of contributions made by employer into fund established for purposes of making future disbursement to individual in accordance with deferred compensation arrangement.

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Amount of cost for defined contribution plan.

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Amount of discretionary contributions made by an employer to a defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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This item represents the amount that could be realized under a life insurance contract or contracts owned by the Entity as of the date of the statement of financial position. Such Entity-owned life insurance policies are commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

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Amount of cost for employer contribution to multiemployer plan. Multiemployer plan includes, but is not limited to, pension plan determined to be individually significant and insignificant and other postretirement benefit plan.

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Percentage of entity's employees covered by collective-bargaining arrangement (CBA) to all participants in multiemployer pension plan determined to be individually significant.

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