Income Taxes - Reconciliation of Total Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of unrecognized tax benefits
Unrecognized tax benefits balance at January 1		$ 1,138	$ 846	$ 552
Gross increase for tax positions taken during a prior year		94	66	89
Gross decrease for tax positions taken during a prior year		(123)	0	(17)
Gross increase for tax positions taken during the current year		132	229	240
Settlement with taxing authorities	$ (437)	(312)	(1)	(18)
Lapse of statute of limitations		(12)	(2)	0
Unrecognized tax benefits balance at December 31		$ 917	$ 1,138	$ 846

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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Amount of unrecognized tax benefits.

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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 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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