Fair Value of Financial Instruments and Fair Value Measurements - Reconciliation of Changes in Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] (Deprecated 2019-01-31)
Fair Value Beginning of Period,	$ 16	$ (32)	$ 25	$ (15)
Gains recognized in operating revenues	2	38	10	42
(Losses) recognized in operating revenues	0	(22)	(4)	(5)
Total gains recognized in operating revenues	2	16	6	37
Gains recognized in OCI	0	12	2	0
(Losses) recognized in OCI	(1)	0	(7)	(2)
Total gains (losses) recognized in OCI	(1)	12	(5)	(2)
Net change recognized in regulatory assets and liabilities	3	1	2	2
Purchases	(1)	0	0	(23)
Settlements	(1)	10	(9)	8
Transfers out of Level 3 (a)	(1)	0	(2)	0
Fair Value Ending of Period,	17	7	17	7
Gains for the period included in operating revenues attributable to the change in unrealized gains relating to financial instruments still held at the reporting date	$ 2	$ 16	$ 6	$ 37

Changes in fair value derivatives recognized in regulatory assets and liabilities, net.

No definition available.

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- Definition

Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Inputs Reconciliation Gain Included In Earnings.

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No definition available.

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Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Inputs Reconciliation Gain Included In Other Comprehensive Income Loss.

+ References

No definition available.

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Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Inputs Reconciliation Loss Included In Earnings.

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No definition available.

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Fair Value Net Derivative Asset Liability Measured On Recurring Basis Unobservable Inputs Reconciliation Loss Included In Other Comprehensive Income Loss.

+ References

No definition available.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of unrealized gain (loss) recognized in income for derivative asset (liability) after deduction of derivative liability (asset), measured at fair value using unobservable input (level 3) and still held.

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- Definition

Amount of gain (loss) recognized in income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

+ References

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- Definition

Amount of gain (loss) recognized in other comprehensive income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

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- Definition

Amount of purchases of financial instrument classified as a 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.

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

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- Definition

Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

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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.

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