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Debt Obligations - Credit Facility Terms under the Amended and Restated Credit Agreement (Detail) - Senior Secured Credit Facility
 23 Months Ended
Nov. 20, 2017
USD ($)
Nov. 19, 2017
Oct. 10, 2018
USD ($)
Line of Credit Facility [Line Items]      
Increase in maximum additional interest rates and fee percentages   0.125%  
Reduction in Quanta's funded indebtedness reduced by cash and cash equivalents in excess of this amount $ 25,000,000.0    
Percentage of capital stock of direct foreign subsidiaries of wholly owned U.S. subsidiaries to secure credit agreement 65.00%    
Maximum consolidated leverage ratio 3.0    
Acquisition threshold for leverage ratio $ 200,000,000.0    
Maximum leverage ratio acquisition completed in current and two subsequent quarters 3.5    
Minimum consolidated interest coverage ratio 3.0    
Amount of availability under the credit agreement and/or cash and cash equivalents on hand that must be present to allow for cash payments of dividends and stock repurchases $ 100,000,000.0    
Cross default provisions with debt instruments exceeding this amount $ 100,000,000.0    
Subsequent Event      
Line of Credit Facility [Line Items]      
Cross default provisions with debt instruments exceeding this amount     $ 150,000,000.0
Minimum      
Line of Credit Facility [Line Items]      
Commitment fee 0.20%    
Maximum      
Line of Credit Facility [Line Items]      
Commitment fee 0.40%    
Standby Letters of Credit | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.125%    
Standby Letters of Credit | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 2.00%    
Performance Letters of Credit | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.675%    
Performance Letters of Credit | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.15%    
Excess of Eurocurrency Rate Applicable to Domestic Borrowings Only | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.125%    
Excess of Eurocurrency Rate Applicable to Domestic Borrowings Only | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 2.00%    
Excess of Base Rate Domestic Borrowings Only | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.125%    
Excess of Base Rate Domestic Borrowings Only | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.00%    
Excess of Euro Currency Rate of Credit Agreement for Foreign Borrowings | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.125%    
Excess of Euro Currency Rate of Credit Agreement for Foreign Borrowings | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 2.00%    
Excess of Federal Funds Rate      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.50%    
Excess of Euro Currency Rate      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.00%