Fair Value Measurements Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale investments	$ 7	$ 8
Derivatives	25	86
Derivatives	(217)	(79)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money market funds	3,826	3,783
Available-for-sale investments	7	8
Derivatives	25	86
Total assets	3,858	3,877
Derivatives	(217)	(79)
Total liabilities	(217)	(79)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money market funds	3,826	3,783
Available-for-sale investments	7	6
Total assets	3,833	3,789
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives	25	86
Total assets	25	86
Derivatives	(217)	(79)
Total liabilities	(217)	(79)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available-for-sale investments		2
Total assets		$ 2