Income Taxes - Reconciliation of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, beginning of period	$ 119	$ 117	$ 87
Decrease related to expiration of statute of limitations	(4)	(3)	(5)
Decrease related to prior years tax positions	(15)	(25)	(1)
Increase related to prior years tax positions	8	15	28
Increase related to current year tax positions	10	15	8
Decrease related to settlements	(11)
Balance, end of period	$ 107	$ 119	$ 117

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

No definition available.

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- Definition

Amount of unrecognized tax benefits.

+ References

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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