Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 1,429,037	$ 1,425,625	$ 3,150,713	$ 2,547,604
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	598,917		1,323,780
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	313,626		729,939
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	516,494		1,096,994
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,406,402		3,085,297
U.S. [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	594,525		1,308,021
U.S. [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	296,156		682,833
U.S. [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	515,721		1,094,443
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	22,635		65,416
Canada [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,392		15,759
Canada [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	17,470		47,106
Canada [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 773		$ 2,551