Fair Value Measurements Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt instruments	$ 491	$ 490
Derivatives, Assets	36	93
Derivatives, Liabilities	(59)	(13)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	1,688	1,582
Commercial Paper, at Carrying Value	121	70
Available for sale debt instruments	346	382
Available for Sale Securities, Government Agencies	31	47
Investments, Fair Value Disclosure	16	18
Derivatives, Assets	36	93
Total assets	2,238	2,192
Derivatives, Liabilities	(59)	(13)
Total liabilities	(59)	(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	1,688	1,582
Investments, Fair Value Disclosure	16	18
Total assets	1,704	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	121	70
Available for sale debt instruments	346	382
Available for Sale Securities, Government Agencies	31	47
Derivatives, Assets	36	93
Total assets	534	592
Derivatives, Liabilities	(59)	(13)
Total liabilities	$ (59)	$ (13)