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Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Instruments (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 116	$ 193	$ 39	$ 4
Contracts added from acquisitions	(2)	0	(3)	12
Total (losses)/gains — realized/unrealized included in earnings	(18)	0	(44)	(15)
Included in OCI	(14)
Cash received	0		(1)
Purchases	38	12	26	9
Transfers into Level 3	(126)	(201)	4	(4)
Transfers out of Level 3	24	2	11	0
Ending balance	18	6	18	6
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held	44	(3)	14	(18)
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	19	19	19
Contracts added from acquisitions	0	0	0	0
Total (losses)/gains — realized/unrealized included in earnings	0	0	1	0
Included in OCI	(14)
Cash received	0		(1)
Purchases	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	5	19	5	19
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held	0	0	1	0
Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	97	174	20	(15)
Contracts added from acquisitions	(2)	0	(3)	12
Total (losses)/gains — realized/unrealized included in earnings	(18)	0	(45)	(15)
Included in OCI	0
Cash received	0		0
Purchases	38	12	26	9
Transfers into Level 3	(126)	(201)	4	(4)
Transfers out of Level 3	24	2	11	0
Ending balance	13	(13)	13	(13)
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held	$ 44	$ (3)	$ 13	$ (18)

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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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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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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 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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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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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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