Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unrecognized Tax Benefits
Gross unrecognized tax benefit balances	$ 16	$ 34	$ 97
Reductions based on settlements with taxing authority	(21)	$ (73)	$ (22)
Unrecognized Tax Benefits, Other Disclosures
Unrecognized tax benefits, if recognized, which would affect effective tax rate	16
Estimate of increase in unrecognized tax benefits in next year	3
Unrecognized tax benefits balance reasonably possible next year	$ 19

Unrecognized tax benefits balance reasonably possible next year

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Amount of decrease reasonably possible in the next twelve months for the unrecognized tax benefit.

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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 unrecognized tax benefits.

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

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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