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Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) Customer Institution Supplier | Dec. 31, 2016 USD ($) Customer | Dec. 31, 2015 USD ($) Customer | Dec. 31, 2010 USD ($)
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Charge to earning to fully reserve the equity investment and note |  |  |  | $ 2,400,000
Maximum term of original maturities to classify as cash equivalent | 3 months |  |  |
Number of financial institutions where deposits are maintained | Institution | 1 |  |  |
Valuation allowance | $ 0 | $ 0 |  |
Number of customers that accounted for 10% or more of net sales | Customer | 2 | 2 | 1 |
Number of customer accounted for 10% or more account receivable | Customer | 3 |  |  |
Number of largest raw material suppliers | Supplier | 4 |  |  |
Excess of the replacement cost of inventory over the LIFO value of inventory | $ 20,100,000 |  |  |
Annual impairment test of goodwill | 0 | $ 0 | $ 0 |
Goodwill | 68,461,000 | 10,523,000 |  |
Valuation allowance | 3,096,000 | 4,061,000 |  |
Research and Development costs | 3,800,000 | 3,700,000 | 1,500,000 |
Prepaid expenses for production costs of advertising | 3,800,000 | 2,400,000 | 800,000 |
Branding Expenses | 31,000,000 | 24,800,000 | 23,400,000 |
Income tax expense (benefit) | 46,791,000 | 34,983,000 | $ 28,689,000 |
Accounting Standards Update 2016-09 [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Income tax expense (benefit) | $ (1,500,000) | $ (1,700,000) |  |
Residential Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 25 years |  |  |
Commercial Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 10 years |  |  |
Trex Signature Railing [Member] | Residential Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 25 years |  |  |
Trex Signature Railing [Member] | Commercial Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 25 years |  |  |
Transcend, Enhance, Select and Universal Fascia Product [Member] | Residential Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 25 years |  |  |
Transcend, Enhance, Select and Universal Fascia Product [Member] | Commercial Use [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Warranty period | 10 years |  |  |
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Minimum [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 10.00% | 10.00% | 10.00% |
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Customer One [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 41.00% | 39.00% | 27.00% |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer One [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 29.00% |  |  |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer Two [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 14.00% |  |  |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer Three [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 11.00% |  |  |
Raw Materials [Member] | Supplier Concentration Risk [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Concentration risk as percentage of total | 33.00% | 33.00% | 35.00% |
Denplax [Member] |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |
Holding company's interest in Denplax | 35.00% |  |  |

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