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Summary of Significant Accounting Policies (Details 2) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Mar. 31, 2018 | Mar. 31, 2018
Accounting Standards Update 2017-07 [Member] |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Effect of change in accounting principal on operating results, retirement benefits | $ 11 | $ 21

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