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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, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4	$ (47)
Total losses realized/unrealized included in earnings
Contracts acquired in XOOM acquisition	12
Included in earnings	(21)	39
Purchases	41	(4)
Contracts reclassified to held-for-sale		4
Transfers into Level 3	5	(1)
Transfers out of Level 3	(2)	13
Ending balance	39	4
Losses 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)	1
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	17
Total losses realized/unrealized included in earnings
Included in earnings		2
Purchases	0	0
Contracts reclassified to held-for-sale
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	19	19
Losses 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	0
Derivative
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(15)	(64)
Total losses realized/unrealized included in earnings
Contracts acquired in XOOM acquisition	12
Included in earnings	(21)	37
Purchases	41	(4)
Contracts reclassified to held-for-sale		4
Transfers into Level 3	5	(1)
Transfers out of Level 3	(2)	13
Ending balance	20	(15)
Losses 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)	$ 1

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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 sales of financial instrument classified as a 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 unrealized gain (loss) recognized in the income statement for a 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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Amount of gain (loss) recognized in the income statement 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 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 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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