Acquisitions - Additional Information (Detail)					12 Months Ended
		Jul. 16, 2018 USD ($) Location	May 01, 2018 USD ($) Location	Jan. 02, 2018 USD ($) State Location $ / shares shares	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($) Branch	Mar. 31, 2018 USD ($)	Oct. 25, 2017 USD ($)
Business Acquisition [Line Items]
Preferred stock, issuance | shares					0	0
Mezzanine equity	[1]				$ 399,200,000	$ 399,200,000
Common stock, par value | $ / shares					$ 0.01	$ 0.01
Number of business locations acquired | Branch								7
Business acquisitions purchase price allocation goodwill amount					$ 2,490,400,000	$ 2,490,600,000		$ 2,491,800,000
Senior Notes, Matures November 2025 [Member]
Business Acquisition [Line Items]
Debt instrument, aggregate principal amount										$ 1,300,000,000
Allied Acquisition [Member]
Business Acquisition [Line Items]
Adjusted purchase price					2,880,000,000		$ 2,877,100,000		$ 2,745,800,000
Net working capital adjustment					88,100,000
Business acquisition purchase price and tax position					164,000,000.0
Number of business locations acquired | Location				208
Number of states business location acquired | State				31
Business combination purchase price tax adjustment to be received, net					5,100,000
Business acquisition, goodwill, tax deductible amount					1,010,000,000.00
Business acquisitions purchase price allocation goodwill amount							1,232,800,000		1,130,600,000
Business acquisitions purchase price allocation intangible assets other than goodwill							$ 1,037,000,000.0		$ 1,037,000,000.0
Allied Acquisition [Member] | Investment Agreement [Member] | Series A Cumulative Convertible Participating Preferred Stock [Member]
Business Acquisition [Line Items]
Preferred stock, issuance | shares				400,000,000,000
Preferred stock, par value | $ / shares				$ 0.01
Preferred stock, liquidation preference value				$ 400,000,000.0
Preferred stock, liquidation purchase price per share | $ / shares				$ 1,000
Proceeds from convertible preferred stock				$ 400,000,000.0
Preferred stock conversion price per share | $ / shares				$ 41.26
Preferred stock dividend rate				6.00%
Mezzanine equity					399,200,000
Unamortized issuance costs					800,000
Allied Acquisition [Member] | Secured Term Loan B Facility [Member]
Business Acquisition [Line Items]
Long-term line of credit				$ 970,000,000.0
Allied Acquisition [Member] | Secured Term Loan B Facility [Member] | LIBOR [Member]
Business Acquisition [Line Items]
Base borrowing rates				1.25%
Allied Acquisition [Member] | Senior Notes, Matures November 2025 [Member]
Business Acquisition [Line Items]
Debt instrument, aggregate principal amount				$ 1,300,000,000
Allied Acquisition [Member] | Senior-secured Asset-based Revolving Line Of Credit [Member] | LIBOR [Member]
Business Acquisition [Line Items]
Base borrowing rates				2.25%
Allied Acquisition [Member] | Stock Purchase Agreement [Member]
Business Acquisition [Line Items]
Acquisition purchase price in cash				$ 2,625,000,000
Tri-State Builder Supply [Member]
Business Acquisition [Line Items]
Number of business locations acquired | Location			1
Business acquisition, sales reported by acquired entity for last annual period			$ 6,000,000
Atlas Supply, Inc. [Member]
Business Acquisition [Line Items]
Number of business locations acquired | Location		6
Business acquisition, sales reported by acquired entity for last annual period		$ 37,000,000
Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition, goodwill, tax deductible amount					$ 6,500,000
Business acquisitions purchase price allocation goodwill amount								7,600,000
Business acquisitions purchase price allocation intangible assets other than goodwill								$ 11,400,000

[1] See Note 3 for additional information.