Acquisitions - Additional Information (Detail)				6 Months Ended
	Jul. 16, 2018 USD ($) Location	May 01, 2018 USD ($) Location	Jan. 02, 2018 USD ($) State Location $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) Branch $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Oct. 25, 2017 USD ($)	Sep. 30, 2017 USD ($)
Business Acquisition [Line Items]
Preferred stock, issuance | shares				0	0	0
Mezzanine equity				$ 399,195,000	$ 399,195,000	$ 399,195,000
Common stock, par value | $ / shares				$ 0.01	$ 0.01	$ 0.01
Number of business locations acquired | Branch					7
Business acquisitions purchase price allocation goodwill amount				$ 2,490,326,000	$ 2,491,779,000	$ 2,381,620,000		$ 1,251,986,000
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]
Acquisition purchase price in cash			$ 2,625,000,000
Adjusted purchase price				2,877,071,000		2,745,882,000
Net working capital adjustment				88,100,000
Business acquisition purchase price and tax position				164,000,000
Number of business locations acquired | Location			208
Number of states business location acquired | State			31
Business acquisition, goodwill, tax deductible amount				1,140,000,000
Business acquisitions purchase price allocation goodwill amount				1,232,780,000		1,130,635,000
Business acquisitions purchase price allocation intangible assets other than goodwill				1,037,000,000		$ 1,037,000,000
Allied Acquisition [Member] | Investment Agreement [Member] | Series A Cumulative Convertible Participating Preferred Stock [Member]
Business Acquisition [Line Items]
Preferred stock, issuance | shares			400,000
Preferred stock, par value | $ / shares			$ 0.01
Preferred stock, liquidation preference value			$ 400,000,000
Preferred stock, liquidation purchase price per share | $ / shares			$ 1,000
Proceeds from convertible preferred stock			$ 400,000,000
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
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				$ 7,200,000
Business acquisitions purchase price allocation goodwill amount					7,600,000
Business acquisitions purchase price allocation intangible assets other than goodwill					$ 11,400,000