Acquisitions - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	6 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Business Combination Separately Recognized Transactions [Line Items]
Goodwill	$ 2,490,326	$ 2,381,620	$ 2,491,779	$ 1,251,986
Goodwill	(513)	1,130,635
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	$ 2,745,882
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

Business combination provisional information initial accounting incomplete adjustment accounts receivable.

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Business combination provisional information initial accounting incomplete adjustment asset liability purchase price.

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Business combination provisional information initial accounting incomplete adjustment current liabilities.

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Business combinaton provisional information initial accounting incomplete adjustment non-current liabilities.

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Business combination provisional information initial accounting incomplete adjustment prepaid and other current assets.

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This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to inventory acquired in connection with a business combination for which the initial accounting was incomplete.

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This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to property, plant, and equipment acquired in connection with a business combination for which the initial accounting was incomplete.

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Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of asset related to consideration paid in advance for costs that provide economic benefits in future periods, and amount of other assets that are expected to be realized or consumed within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount due from customers or clients for goods or services, including trade receivables, that have been delivered or sold in the normal course of business, and amounts due from others, including related parties expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount of liabilities due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

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The amount of inventory recognized as of the acquisition date.

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Amount of liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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