Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	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%