Acquisitions (Detail) - USD ($) $ in Thousands			3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jul. 20, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Potential payment (up to)	$ 157,300		$ 157,300	$ 157,300	$ 157,300
Net tangible assets acquired					37,500
Other intangible assets acquired					52,400
Goodwill acquired				55,049	55,000
Electric Power Infrastructure Services Division
Business Acquisition [Line Items]
Goodwill acquired				55,049
Oil and Gas Infrastructure Services Division
Business Acquisition [Line Items]
Goodwill acquired				0
Acquisitions 2018
Business Acquisition [Line Items]
Cash consideration	105,545			$ 105,500
Number of shares granted for acquired companies (in shares)				679,668
Value of Quanta common stock issued	22,882			$ 22,900
Potential payment (up to)	18,000		18,000	18,000	18,000
Fair value of contingent consideration liability	16,471		16,471	16,471	16,471
Other intangible assets acquired				52,364
Goodwill acquired	55,049
Goodwill expected to be deductible for income tax	21,600		21,600	21,600	21,600
Revenues included in consolidated results of operations			12,900	32,200
Income (loss) before taxes			(5,100)	(11,700)
Acquisition costs			5,000	$ 11,000
Acquisitions 2018 | Minimum
Business Acquisition [Line Items]
Contingent consideration financial target term				3 years
Acquisitions 2018 | Maximum
Business Acquisition [Line Items]
Contingent consideration financial target term				5 years
Acquisitions 2018 | Electric Power Infrastructure Services Division
Business Acquisition [Line Items]
Goodwill acquired				$ 55,000
Stronghold, Ltd. and Stronghold Specialty, Ltd.
Business Acquisition [Line Items]
Cash consideration	351,014	$ 351,000
Number of shares granted for acquired companies (in shares)		2,693,680
Value of Quanta common stock issued	81,337	$ 81,300
Potential payment (up to)		$ 100,000
Contingent consideration financial target term		3 years
Fair value of contingent consideration liability		$ 51,100					$ 51,084
Goodwill acquired	296,542
Stronghold, Ltd. and Stronghold Specialty, Ltd. | Oil and Gas Infrastructure Services Division
Business Acquisition [Line Items]
Goodwill acquired						$ 296,500
Acquisitions 2017, other than Stronghold
Business Acquisition [Line Items]
Cash consideration	11,955					$ 12,000
Number of shares granted for acquired companies (in shares)						288,666
Value of Quanta common stock issued	8,267					$ 8,300
Fair value of contingent consideration liability	0		$ 0	$ 0	$ 0
Goodwill acquired	$ 5,917
Acquisitions 2017, other than Stronghold | Electric Power Infrastructure Services Division
Business Acquisition [Line Items]
Goodwill acquired						5,900
Acquisitions 2017
Business Acquisition [Line Items]
Goodwill expected to be deductible for income tax						$ 302,500