Acquisitions - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2019	Sep. 30, 2019		Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017
Business Combination Separately Recognized Transactions [Line Items]
Goodwill		$ 2,490,590		$ 2,491,779		$ 1,251,986
Goodwill		(513)	[1]	$ 1,240,913
Allied Acquisition [Member]
Business Combination Separately Recognized Transactions [Line Items]
Cash	$ 169				$ 19,322
Accounts receivable	337,549				315,485
Inventory	314,785				322,705
Prepaid and other current assets	75,440				59,279
Property, plant, and equipment	139,360				139,528
Goodwill	1,232,780				1,130,635
Intangible assets	1,037,000				1,037,000
Current liabilities	(259,289)				(271,252)
Non-current liabilities	(723)				(6,820)
Total purchase price	2,877,071	$ 2,880,000			$ 2,745,882
Cash	(19,153)
Accounts receivable	22,064
Inventory	(7,920)
Prepaid and other current assets	16,161
Property, plant, and equipment	(168)
Goodwill	102,145
Current liabilities	11,963
Non-current liabilities	6,097
Total purchase price	$ 131,189

[1] Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion).