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Employee Benefit Plans - Assumptions Used to Determine Pension Benefit Obligations and Periodic Benefit Costs, Postretirement Benefit Obligations and Net Costs (Parenthetical) (Detail) | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
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Postretirement Benefits [Member] | U.S. Defined Benefit Plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Initial health care cost trend rate | 5.00% | [1] | 0.00% | 5.50% | [1]
[1] | The initial health care cost trend rate is set at 5% for the 2023 U.S. plan assumptions as the 2024 Medicare Part B premiums are expected to increase. The trend rates for all future years beyond 2024 also reflect expected 5% increases. The initial health care cost trend rate was set at zero for the 2022 U.S. plan assumptions in light of the expected downward adjustment the U.S. government was expected to make to the 2023 Medicare Part B premiums, however, all trend rates beyond the initial year were set to 5%

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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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