Income Taxes - Rollforward of Total Unrecognized Tax Liabilities (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	$ 2,181	$ 1,852	$ 1,350
Tax Positions Related to Current Year: Additions	64	299	522
Tax Positions Related to Current Year: Reductions	0	0	0
Tax Positions Related to Prior Years: Additions	125	67	33
Tax Positions Related to Prior Years: Reductions	0	(16)	(3)
Settlements	(774)	(12)	(49)
Lapse of statute of limitations	(1)	(9)	(1)
Balance, end of period	$ 1,595	$ 2,181	$ 1,852

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.

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

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