Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Fair Value Measurements (Detail)

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Summary of Significant Accounting Policies - Fair Value Measurements (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Jan. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Acquisitions that have maximum payouts $ 154,400,000     $ 154,400,000      
Increase (decrease) in fair value of contingent consideration liabilities (6,279,000) $ 0   (6,279,000) $ 0    
Charge associated with disposition of construction barge 3,300,000   $ 1,900,000        
Acquisitions 2018              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Acquisitions that have maximum payouts           $ 15,000,000  
Contingent consideration, liability 13,705,000     13,705,000   $ 13,700,000  
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Acquisition Without Maximum Earn-out              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Contingent consideration, liability 1,000,000     1,000,000      
Contingent consideration | Fair Value, Inputs, Level 3              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Contingent consideration, liability 73,000,000     73,000,000     $ 65,700,000
Contingent consideration, acquisitions       13,700,000      
Contingent consideration, payments 0 0   0 0    
Increase (decrease) in fair value of contingent consideration liabilities $ (6,300,000) $ 0   $ (6,300,000) $ 0    
Contingent consideration | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Minimum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value inputs expected volatility rate       23.00%      
Fair value inputs discount rate       2.10%      
Contingent consideration | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Maximum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value inputs expected volatility rate       30.00%      
Fair value inputs discount rate       3.40%