Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 1,924,534	$ 1,934,951	$ 5,075,247	$ 4,482,555
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,870,923		4,956,220
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	53,611		119,027
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	848,259		2,168,755
Residential Roofing Products [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	829,643		2,134,328
Residential Roofing Products [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	18,616		34,427
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	472,105		1,200,546
Non-Residential Roofing Products [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	439,516		1,120,885
Non-Residential Roofing Products [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	32,589		79,661
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	604,170		1,705,946
Complementary Building Products [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	601,764		1,701,007
Complementary Building Products [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 2,406		$ 4,939