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Business Combinations - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Acquisition [Line Items]      
Percentage of voting equity interests acquired 100.00%    
Goodwill acquired expected to be tax deductible $ 47,200    
Total Purchase Price 153,354 $ 59,952 $ 98,380
Gain on bargain purchase     1,116
Amortization of intangibles 26,857 11,259 6,264
Income tax expense (benefit) 14,680 21,174 15,413
Interest expense 17,381 6,177 3,738
Administrative [Member]      
Business Acquisition [Line Items]      
Acquisition-related costs 3,900 2,300 1,100
Alpha [Member]      
Business Acquisition [Line Items]      
Increase in goodwill 2,100    
Fair value of assets acquired 116,671    
Total Purchase Price 116,671    
Columbia [Member]      
Business Acquisition [Line Items]      
Increase in goodwill 500    
Increase in seller obligations 400    
Increase in purchase price 600    
Fair value of assets acquired 8,993    
Total Purchase Price 8,993    
Combined Business Acquisitions [Member]      
Business Acquisition [Line Items]      
Fair value of assets acquired 153,354 59,952 99,496
Gain on bargain purchase     1,116
Amortization of intangibles 1,300 17,500 6,600
Income tax expense (benefit) 700 3,000 1,700
Interest expense   $ 1,800  
Business Combination Included as "Other" [Member]      
Business Acquisition [Line Items]      
Fair value of assets acquired     4,800
Total Purchase Price     3,700
Gain on bargain purchase     1,100
BDI Insulation [Member]      
Business Acquisition [Line Items]      
Fair value of assets acquired     36,445
Total Purchase Price     36,445
Insurance receivable for claims under a fully insured policy     2,000
Indemnification asset associated with the acquisition $ 900   $ 1,700