Acquisitions (2014 Acquisitions - Pacer) (Narrative) (Details)	1 Months Ended	3 Months Ended	7 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Dec. 31, 2014 CAD	Dec. 31, 2014 USD ($)	Jun. 01, 2014 USD ($)
Business Combinations [Line Items]
Other current liabilities, equity method investments (in dollars)							$ 34,341,000		$ 66,527,000
Prepaid expenses and other current assets, equity method investments							116,985,000		91,336,000
Performance Guarantees [Member]
Business Combinations [Line Items]
Guarantees issued, related parties							$ 553,700,000		748,300,000
Pacer [Member]
Business Combinations [Line Items]
Business combinations, effective date				Jun. 01, 2014
Business combinations, percentage of voting interests acquired						100.00%	100.00%
Business combinations, name of acquired entity				Pacer Construction Holdings Corporation and its affiliated operating companies (collectively, “Pacer”)
Business combinations, description of acquired entity				Pacer is a western Canadian civil construction services company, headquartered in Calgary, Alberta, Canada.  Pacer’s services include infrastructure construction primarily in support of oil and gas production, processing, mining and transportation.
Business combinations, goodwill recognized, description				Goodwill arising from the acquisition represents the estimated value of Pacer’s geographic presence in key high-growth Canadian markets, its assembled workforce, its management team’s industry-specific project management expertise and synergies expected to be achieved from the combined operations of Pacer and MasTec.
Business combinations, goodwill, expected tax deductible amount										$ 0
Pacer [Member] | Earn-Out Arrangements [Member]
Business Combinations [Line Items]
Business combinations, contingent consideration arrangements, basis for amount				The contingent consideration included in the table above equals 25% of the excess, if any, of Pacer’s earnings from continuing operations before interest, taxes, depreciation and amortization (“EBITDA”) above certain thresholds for a five-year period, as set forth in the purchase agreement, and is payable annually in Canadian dollars.
Business combinations, contingent consideration, percentage of excess EBITDA				25.00%
Business combinations, contingent consideration, earn-out period (in years)				5 years
Business combinations, contingent consideration, fair value measurements, significant assumptions				The fair value of the earn-out liability was estimated using an income approach and incorporates significant inputs not observable in the market. Key assumptions in the estimated valuation include the discount rate and probability-weighted EBITDA projections.
Business combination, contingent consideration arrangements, range of outcomes, value, low | CAD						CAD 0
Business combination, contingent consideration arrangements, range of outcomes, value, high | CAD						71,000,000
Business combination, contingent consideration arrangements, range of outcomes, maximum unlimited				there is no maximum earn-out payment amount
Pacer [Member] | Joint Venture [Member] | Performance Guarantees [Member]
Business Combinations [Line Items]
Guarantees issued, related parties | CAD						CAD 132,100,000		CAD 132,100,000
Pacer [Member] | Joint Venture [Member] | Other [Member] | Performance Guarantees [Member]
Business Combinations [Line Items]
Construction projects, percentage complete						65.00%	65.00%
Pacer [Member] | Equity Method Investee [Member] | Pacer Equity Method Investee in Receivership [Member]
Business Combinations [Line Items]
Equity method investment, expected collection period (in years)				12 months
Pacer [Member] | Equity Method Investee [Member] | Additional Pacer Equity Method Investee in Receivership [Member]
Business Combinations [Line Items]
Equity method investment, expected collection period (in years)				12 months
Pacer [Member] | Equity Method Investee [Member] | Financial Support [Member] | Pacer Equity Method Investee in Receivership [Member]
Business Combinations [Line Items]
Unconsolidated affiliates, outstanding commitments | CAD						CAD 78,000,000
Pacer [Member] | Canadian Dollars [Member]
Business Combinations [Line Items]
Business combinations, provisional information, initial accounting Incomplete, adjustment, property and equipment					$ 8,400,000
Business combinations, provisional information, initial accounting incomplete, adjustment, goodwill					47,500,000
Pacer [Member] | Canadian Dollars [Member] | Earn-Out Arrangements [Member]
Business Combinations [Line Items]
Business combination, contingent consideration arrangements, range of outcomes, value, low							$ 0
Business combination, contingent consideration arrangements, range of outcomes, value, high							54,000,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investments [Member]
Business Combinations [Line Items]
Net equity method investment obligations, financing arrangements										31,000,000
Pacer [Member] | Canadian Dollars [Member] | Current Liabilities [Member]
Business Combinations [Line Items]
Business combinations, provisional information, initial accounting incomplete, adjustment, liabilities					5,600,000
Pacer [Member] | Canadian Dollars [Member] | Long-Term Deferred Tax Liabilities [Member]
Business Combinations [Line Items]
Business combinations, provisional information, initial accounting incomplete, adjustment, liabilities					1,200,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investments [Member]
Business Combinations [Line Items]
Business combinations, provisional information, initial accounting incomplete, adjustment, assets					$ (49,000,000)
Pacer [Member] | Canadian Dollars [Member] | Financial Support [Member] | Equity Method Investments [Member]
Business Combinations [Line Items]
Net equity method investment obligations, financing arrangements										$ 8,600,000
Pacer [Member] | Canadian Dollars [Member] | Joint Venture [Member]
Business Combinations [Line Items]
Revenue, related parties, affiliates		$ 300,000		$ 1,800,000
Pacer [Member] | Canadian Dollars [Member] | Joint Venture [Member] | Financial Support [Member]
Business Combinations [Line Items]
Financing commitments, amounts paid				8,200,000
Unconsolidated affiliates, outstanding commitments							0		0
Pacer [Member] | Canadian Dollars [Member] | Joint Venture [Member] | Accounts Receivable [Member]
Business Combinations [Line Items]
Receivables, related parties, unincorporated joint venture							1,100,000		1,800,000
Pacer [Member] | Canadian Dollars [Member] | Joint Venture [Member] | Performance Guarantees [Member]
Business Combinations [Line Items]
Guarantees issued, related parties							99,200,000		113,600,000
Pacer [Member] | Canadian Dollars [Member] | Joint Venture [Member] | Other [Member] | Contracts Accounted for under Percentage-of-Completion [Member]
Business Combinations [Line Items]
Project losses (in dollars)		2,800,000		8,300,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member]
Business Combinations [Line Items]
Revenue, related parties, affiliates		1,600,000		3,100,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Additional Pacer Equity Method Investee in Receivership [Member] | Maximum [Member]
Business Combinations [Line Items]
Equity method investment, net assets							1,000,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Financial Support [Member]
Business Combinations [Line Items]
Financing commitments, amounts paid				$ 10,200,000
Unconsolidated affiliates, outstanding commitments							0		0
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Financial Support [Member] | Pacer Equity Method Investee in Receivership [Member]
Business Combinations [Line Items]
Financing commitments, amounts paid			$ 38,300,000
Unconsolidated affiliates, outstanding commitments							58,600,000
Unconsolidated affiliates, financing commitments, amounts repaid		$ 12,800,000
Prepaid expenses and other current assets, equity method investments							23,100,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Accounts Receivable [Member]
Business Combinations [Line Items]
Receivables, related parties, equity method investee							1,600,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Performance Guarantees [Member]
Business Combinations [Line Items]
Guarantees issued, related parties							30,700,000		61,200,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Other Financial Guarantees [Member]
Business Combinations [Line Items]
Guarantees issued, related parties							$ 2,200,000		3,000,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Guarantee of Indebtedness of Others [Member] | Current Liabilities [Member]
Business Combinations [Line Items]
Other current liabilities, equity method investments (in dollars)									$ 22,600,000
Pacer [Member] | Canadian Dollars [Member] | Equity Method Investee [Member] | Guarantee of Indebtedness of Others [Member] | Financial Support [Member] | Revolving Loans [Member]
Business Combinations [Line Items]
Financing commitments, amounts paid	$ 20,800,000