Revenue Energy Prepayment Agreement (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Long-term Contract for Purchase of Electric Power [Line Items]
Proceeds from Maturities, Prepayments and Calls of Other Investments	$ 0	$ 1,500
Other Significant Noncash Transaction, Value of Consideration Given		10	$ 50	$ 100
Discount to Revenue [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	$ 12	$ 4	$ 23

The value of the noncash (or part noncash) consideration given (for example, liability, equity) in a transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of a transaction not resulting in cash receipts or cash payments in the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration