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Income Taxes - Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of period		$ 1,183	$ 486	$ 555
Additions based on tax positions related to prior years		78	47	7
Reductions based on tax positions related to prior years		(234)	(124)	(67)
Additions based on tax positions related to current year		68	778	105
Reductions based on settlements	$ (39)	(13)	(7)	(113)
Reductions based on the lapse of the applicable statutes of limitations		(25)	0	0
Exchange rate fluctuations		(5)	3	(1)
Unrecognized tax benefits at end of period		$ 1,052	$ 1,183	$ 486

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Unrecognized Tax Benefits, Increase (Decrease) Resulting from Foreign Currency Translation

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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 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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