Revenue Energy Prepayment Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2004
Energy Prepayment Obligation [Line Items]
Electric revenue	$ 10,104	$ 11,159	$ 11,075
Energy Prepayment Discount	$ 4	$ 46
MLGW
Energy Prepayment Obligation [Line Items]
Deferred Revenue, Description	10 million	100 million		100 million

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=121566466&loc=d3e6935-107765

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