Goodwill and Other Intangible Assets - Summary of Changes in Quanta's Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Goodwill gross, beginning balance			$ 1,642,902	$ 1,592,551
Accumulated impairment, beginning balance			(39,733)	(39,893)
Goodwill net, beginning balance			1,603,169	1,552,658
Goodwill acquired			302,408	45,186
Purchase price allocation adjustments			(1,278)	15
Goodwill impairments	$ (57,000)	$ (39,800)	(57,011)
Foreign currency translation adjustments			21,312	5,310
Goodwill gross, ending balance	1,966,432	1,592,551	1,966,432	1,642,902
Accumulated impairment, ending balance	(97,832)	(39,893)	(97,832)	(39,733)
Goodwill net, ending balance	1,868,600	1,552,658	1,868,600	1,603,169
Electric Power Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance			1,253,979	1,226,245
Accumulated impairment, beginning balance			0	0
Goodwill net, beginning balance			1,253,979	1,226,245
Goodwill acquired			5,866	24,168
Purchase price allocation adjustments			(619)	229
Goodwill impairments			0
Foreign currency translation adjustments			13,301	3,337
Goodwill gross, ending balance	1,272,527	1,226,245	1,272,527	1,253,979
Accumulated impairment, ending balance	0	0	0	0
Goodwill net, ending balance	1,272,527	1,226,245	1,272,527	1,253,979
Oil and Gas Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance			388,923	366,306
Accumulated impairment, beginning balance			(39,733)	(39,893)
Goodwill net, beginning balance			349,190	326,413
Goodwill acquired			296,542	21,018
Purchase price allocation adjustments			(659)	(214)
Goodwill impairments			(57,011)
Foreign currency translation adjustments			8,011	1,973
Goodwill gross, ending balance	693,905	366,306	693,905	388,923
Accumulated impairment, ending balance	(97,832)	(39,893)	(97,832)	(39,733)
Goodwill net, ending balance	$ 596,073	$ 326,413	$ 596,073	$ 349,190

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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