Acquisitions - Additional Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 01, 2016	May 02, 2016	Apr. 01, 2016	Dec. 29, 2015	Dec. 18, 2015	Dec. 01, 2015	Oct. 31, 2015
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period		$ 3,707,629
Business Acquisitions Purchase Price Allocation Goodwill Amount	$ 84,800
Business Acquisitions Purchase Price Allocation Intangible Assets Other Than Goodwill	60,800
Business Combination Good will Tax Deductible Portion	86,100
Revenue, Net	4,127,109	$ 2,515,169	$ 2,326,905
Business Combination, Acquired Receivables, Fair Value	177,300
Business Combination, Acquired Receivables, Gross Contractual Amount	$ 185,900
Number Of Business Locations	42
Business Acquisition, Goodwill, Expected Tax Deductible Amount	$ 59,800
RCI Roofing Supply [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	23,000
Number Of Business Locations									5
Roofing and Insulation Supply [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	70,000
Business Acquisition, Percentage of Voting Interests Acquired								100.00%
Number Of Business Locations								20
Statewide Wholesale [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	$ 15,000
Number Of Business Locations							1
Roofing Supply Groups [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Business Combination, Consideration Transferred, Total	$ 1,170,000
Payments to Acquire Businesses, Gross	288,200
Business Combination, Consideration Transferred, Other	306,800
Business Combination, Consideration Transferred, Liabilities Incurred	$ 574,400
Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Grants In Period | shares	661,349
Business Acquisition, Transaction Costs	$ 58,200
Revenue, Net	1,400,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	10,300
RSG Acquisition [Member]
Business Acquisition [Line Items]
Number Of Business Locations										85
Number Of States Business Location										25
Atlantic Building Product [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	5,000
Number Of Business Locations						2
Lyf-Tym Building Product [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	20,000
Number Of Business Locations						6
Fox Brothers [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	35,000
Number Of Business Locations					4
Woodfeathers, inc. [Member]
Business Acquisition [Line Items]
Business Acquisition, sales reported by acquired entity for last annual period	$ 30,000
Business Acquisition, Percentage of Voting Interests Acquired				100.00%
Number Of Business Locations				4