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Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Balance Sheets (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Sep. 30, 2017 | Oct. 01, 2016
--- | --- | --- | ---
Significant Accounting Policies [Line Items] |  |  |
Inventories | $ 482 | $ 433 |
Deferred income taxes (assets) | 68 | 237 |
Retained earnings | $ 1,417 | 1,707 |
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Inventories |  | 433 |
Deferred income taxes (assets) |  | 237 |
Retained earnings |  | 1,707 | $ 1,561
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | As Originally Reported [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Inventories |  | 396 |
Deferred income taxes (assets) |  | 250 |
Retained earnings |  | 1,683 | 1,544
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | Effect of Change [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Inventories |  | 37 |
Deferred income taxes (assets) |  | (13) |
Retained earnings |  | $ 24 | $ 17

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