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Subsequent Events (Details) - USD ($)
$ in Millions
1 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Subsequent Event [Line Items]    
Contingent consideration, maximum   $ 139.5
Electrical Infrastructure Services Business and Postsecondary Educational Institution | Subsequent Event    
Subsequent Event [Line Items]    
Aggregate consideration paid $ 47.9  
Number of shares granted for acquired companies (in shares) 379,817  
Value of Quanta common stock issued $ 13.6  
Contingent consideration, maximum $ 15.0