Goodwill and Other Intangible Assets - Summary of Changes in Quanta's Goodwill (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Goodwill [Roll Forward]
Goodwill gross, beginning balance	$ 1,966,432
Accumulated impairment, beginning balance	(97,832)
Goodwill, beginning balance	1,868,600
Goodwill acquired	43,181	$ 344,200
Purchase price allocation adjustments	51
Foreign currency translation adjustments	(14,168)
Goodwill gross, ending balance	1,994,776	1,994,776
Accumulated impairment, ending balance	(97,112)	(97,112)
Goodwill, ending balance	1,897,664	1,897,664
Electric Power Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance	1,272,527
Accumulated impairment, beginning balance	0
Goodwill, beginning balance	1,272,527
Goodwill acquired	43,181
Purchase price allocation adjustments	51
Foreign currency translation adjustments	(8,772)
Goodwill gross, ending balance	1,306,987	1,306,987
Accumulated impairment, ending balance	0	0
Goodwill, ending balance	1,306,987	1,306,987
Oil and Gas Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance	693,905
Accumulated impairment, beginning balance	(97,832)
Goodwill, beginning balance	596,073
Goodwill acquired	0
Purchase price allocation adjustments	0
Foreign currency translation adjustments	(5,396)
Goodwill gross, ending balance	687,789	687,789
Accumulated impairment, ending balance	(97,112)	(97,112)
Goodwill, ending balance	$ 590,677	$ 590,677

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
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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