Revenue Energy Prepayment Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2004
Energy Prepayment Obligation [Line Items]
Electric revenue	$ 11,159	$ 11,075	$ 10,586
Energy Prepayment Discount	$ 4	$ 46
MLGW
Energy Prepayment Obligation [Line Items]
Deferred Revenue, Description	10	100		100

Description of the transaction or nature of business giving rise to deferred revenue or unearned revenue, other than annual membership fees. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118951113&loc=d3e6935-107765

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