Acquisitions - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 20, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Contingent consideration payments (up to)		$ 157,200
Goodwill		2,022,675	$ 1,899,879	$ 1,868,600	$ 1,603,169
2019 Acquisitions
Business Acquisition [Line Items]
Cash consideration		$ 399,300
Number of shares granted for acquired companies (in shares)		60,860
Value of Quanta common stock issued		$ 1,800
Net tangible assets acquired		101,000
Identifiable intangible assets		192,800
Goodwill		110,400
Bargain purchase gain		3,100
Goodwill expected to be deductible for income tax purposes		90,600
Revenues included in consolidated results of operations		223,300
Income from continuing operations before income taxes included in consolidated results of operations		7,500
Acquisition-related costs		22,100
Hallen
Business Acquisition [Line Items]
Identifiable intangible assets		$ 175,000
2018 Acquisitions
Business Acquisition [Line Items]
Cash consideration			$ 108,307
Number of shares granted for acquired companies (in shares)			679,668
Value of Quanta common stock issued			$ 22,882
Contingent consideration payments (up to)			18,000
Fair value of contingent consideration liability			16,471
Identifiable intangible assets			52,364
Bargain purchase gain			0
Goodwill expected to be deductible for income tax purposes			21,600
Revenues included in consolidated results of operations			71,100
Income from continuing operations before income taxes included in consolidated results of operations			(8,900)
Acquisition-related costs			$ 11,000
2018 Acquisitions | Minimum
Business Acquisition [Line Items]
Contingent consideration financial target term			5 years
2018 Acquisitions | Maximum
Business Acquisition [Line Items]
Contingent consideration financial target term			3 years
2017 Acquisitions
Business Acquisition [Line Items]
Goodwill expected to be deductible for income tax purposes				251,400
Revenues included in consolidated results of operations				207,400
Income from continuing operations before income taxes included in consolidated results of operations				(8,100)
Acquisition-related costs				$ 5,400
Stronghold
Business Acquisition [Line Items]
Number of shares granted for acquired companies (in shares)	2,693,680
Value of Quanta common stock issued	$ 81,300
Contingent consideration payments (up to)	$ 100,000
Contingent consideration financial target term	3 years
Fair value of contingent consideration liability	$ 51,100
Cash paid or payable	$ 351,000
Other 2017 Acquisitions
Business Acquisition [Line Items]
Number of shares granted for acquired companies (in shares)				288,666
Value of Quanta common stock issued				$ 8,300
Cash paid or payable				$ 12,000