Income Taxes - Unrecognized Tax Positions (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in unrecognized tax positions
Balance, beginning of year	$ 127	$ 56	$ 11
Gross increases as a result of tax positions taken during prior periods	1	72	45
Gross decreases as a result of tax positions taken during prior periods	0	0	0
Gross increases as a result of tax positions taken during current period	0	0	2
Balance, end of year	126	127	56
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$ (2)	$ (1)	$ (2)

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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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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