Acquisitions - Aggregate Consideration Paid or Payable and Allocation of Net Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 110,383	$ 56,337	$ 302,408
Hallen
Business Acquisition [Line Items]
Identifiable intangible assets	175,000
Other Acquisitions
Business Acquisition [Line Items]
Cash paid or payable	399,275
Value of Quanta common stock issued	1,791
Contingent consideration	0
Fair value of total consideration transferred or estimated to be transferred	401,066
Accounts receivable	112,142
Contract assets	11,869
Other current assets	14,290
Property and equipment	60,133
Other assets	149
Identifiable intangible assets	192,786
Contract liabilities	(11,856)
Other current liabilities	(73,948)
Deferred tax liabilities, net	(6,398)
Other long-term liabilities	(5,345)
Total identifiable net assets	293,822
Goodwill	110,383
Fair value of net assets acquired	404,205
Bargain purchase gain	$ (3,139)
2018 Acquisitions
Business Acquisition [Line Items]
Cash paid or payable		108,307
Value of Quanta common stock issued		22,882
Contingent consideration		16,471
Fair value of total consideration transferred or estimated to be transferred		147,660
Accounts receivable		18,405
Contract assets		1,905
Other current assets		8,484
Property and equipment		23,674
Other assets		576
Identifiable intangible assets		52,364
Contract liabilities		(175)
Other current liabilities		(11,205)
Deferred tax liabilities, net		(4,208)
Other long-term liabilities		0
Total identifiable net assets		89,820
Goodwill		57,840
Fair value of net assets acquired		147,660
Bargain purchase gain		$ 0