Impact of New Accounting Standards and Interpretations Impact of New Accounting Standards and Interpretations (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from Operating and maintenance expense to Other income	$ 758	$ 179	$ 185
Reclassification of net periodic benefit costs		256	758
Reclassification of cash receipts (payments)		$ 29	$ 9

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.

No definition available.

Name:	us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration