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New Accounting Pronouncements - Additional Information (Detail) - USD ($) | Mar. 31, 2019 | Sep. 30, 2018
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Five Year Unsecured Revolving Credit Facility [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Credit facility, borrowing capacity |  | $ 3,000,000,000
Scenario, Forecast [Member] | Five Year Unsecured Revolving Credit Facility [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Credit facility, borrowing capacity | $ 3,000,000,000 |
Scenario, Forecast [Member] | Minimum [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Lease assets | 500,000,000 |
Lease liabilities | 500,000,000 |
Scenario, Forecast [Member] | Maximum [Member] |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Lease assets | 750,000,000 |
Lease liabilities | $ 750,000,000 |

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Balance Type: | credit
Period Type: | instant

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- Definition 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. + References No definition available. + Details
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- Definition Present value of lessee's discounted obligation for lease payments from operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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- Details
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- Details
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- Details
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