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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity contract derivatives		$ 32	$ 49
Commodity contract derivatives		5	3
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity contract derivatives		0	0
Commodity contract derivatives		0	0
Commodity Contract Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance	$ (7)		$ 0	$ (17)	$ (4)
Net unrealized gains (losses) deferred as regulatory assets and liabilities	$ 10	$ (3)

X

- Definition

The fair value as of the balance sheet date of firm obligations in physical commodity contracts (for example, agricultural products, metals and energy).

+ References

No definition available.

+ Details

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X

- Definition

This item represents the amount of the total realized and unrealized gains or losses for the period which are deferred as regulatory assets and liabilities; the fair value of which assets or liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3).

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

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- Definition

Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

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X

- Definition

The fair value as of the balance sheet date of firm holdings in physical commodity contracts (for example, agricultural products, metals and energy). Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

No definition available.

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