Fair Value Measurements Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 478	$ 490
Derivatives, Assets	106	93
Derivatives, Liabilities	(12)	(13)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	1,983	1,582
Commercial Paper, at Carrying Value	126	70
Available-for-sale Securities, Debt Securities	340	382
Available for Sale Securities, Government Agencies	33	47
Investments, Fair Value Disclosure	17	18
Derivatives, Assets	106	93
Total assets	2,605	2,192
Derivatives, Liabilities	(12)	(13)
Total liabilities	(12)	(13)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	1,983	1,582
Investments, Fair Value Disclosure	17	18
Total assets	2,000	1,600
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial Paper, at Carrying Value	126	70
Available-for-sale Securities, Debt Securities	340	382
Available for Sale Securities, Government Agencies	33	47
Derivatives, Assets	106	93
Total assets	605	592
Derivatives, Liabilities	(12)	(13)
Total liabilities	$ (12)	$ (13)