Acquisitions (2014 Acquisitions) - Additional Information (Detail) - Acquisitions 2014 [Member] $ in Millions	12 Months Ended
Dec. 31, 2014 USD ($) Entity shares
Business Acquisition [Line Items]
Number of business acquisitions	9
Cash paid or payable \| $	$ 279.5
Value of Quanta common stock and exchangeable shares issued \| $	$ 134.5
Common Stock [Member]
Business Acquisition [Line Items]
Number of shares granted for acquired companies \| shares	686,382
Canadian Subsidiaries [Member] \| Exchangeable Shares [Member]
Business Acquisition [Line Items]
Number of shares granted for acquired companies \| shares	3,825,971
Series G Preferred Stock [Member]
Business Acquisition [Line Items]
Number of shares granted for acquired companies \| shares	1
Series G Preferred Stock [Member] \| Canadian Subsidiaries [Member] \| Exchangeable Shares [Member]
Business Acquisition [Line Items]
Number of shares granted for acquired companies \| shares	899,858
Electric Power Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions	1
Electric Power Infrastructure Services Business [Member] \| Australia [Member]
Business Acquisition [Line Items]
Number of business acquisitions	1
Electric Power Infrastructure Services Business [Member] \| Canada [Member]
Business Acquisition [Line Items]
Number of business acquisitions	4
Oil and Gas Infrastructure Services Business [Member] \| United States [Member]
Business Acquisition [Line Items]
Number of business acquisitions	1
Oil and Gas Infrastructure Services Business [Member] \| Canada [Member]
Business Acquisition [Line Items]
Number of business acquisitions	2

Consideration transferred or to be transferred cash and liabilities incurred except for contingent consideration.

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Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

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