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Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Instruments (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 38	$ 39
Contracts added from acquisitions		(3)
Total (losses) — realized/unrealized:
Total (losses) — realized/unrealized included in earnings		(26)
Purchases		40
Sale		(19)
Transfers into Level 3		2
Transfers out of Level 3		5
Ending balance		38
Gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period		17
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	19
Contracts added from acquisitions		0
Total (losses) — realized/unrealized:
Total (losses) — realized/unrealized included in earnings		0
Purchases		0
Sale		(19)
Transfers into Level 3		0
Transfers out of Level 3		0
Ending balance		0
Gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period		0
Derivative
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	38	20
Contracts added from acquisitions		(3)
Total (losses) — realized/unrealized:
Total (losses) — realized/unrealized included in earnings	(44)	(26)
Purchases	(13)	40
Sale		0
Transfers into Level 3	1	2
Transfers out of Level 3	2	5
Ending balance	(16)	38
Gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period	$ 9	$ 17

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Fair Value, Net Derivative Asset (Liability) Measured On Recurring Basis, Unobservable Inputs Reconciliation, Acquisition

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

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
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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).

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

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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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