Fair Value Measurements and Financial Instruments - Schedule of assets and liabilities that are measured at fair value on a recurring basis (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Money market funds
Assets:
Money market funds	$ 1,847	$ 1,371
Government bonds
Assets:
Other investments:	1	1
Equity investments
Assets:
Other investments:	4	9
Gross Foreign exchange contracts
Assets:
Derivatives	33	15
Liabilities:
Derivatives	6	14
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Money market funds	1,847	1,371
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government bonds
Assets:
Other investments:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments
Assets:
Other investments:	0	6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Gross Foreign exchange contracts
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Money market funds	0	0
Significant Other Observable Inputs (Level 2) | Government bonds
Assets:
Other investments:	1	1
Significant Other Observable Inputs (Level 2) | Equity investments
Assets:
Other investments:	4	3
Significant Other Observable Inputs (Level 2) | Gross Foreign exchange contracts
Assets:
Derivatives	33	15
Liabilities:
Derivatives	6	14
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Money market funds	0	0
Significant Unobservable Inputs (Level 3) | Government bonds
Assets:
Other investments:	0	0
Significant Unobservable Inputs (Level 3) | Equity investments
Assets:
Other investments:	0	0
Significant Unobservable Inputs (Level 3) | Gross Foreign exchange contracts
Assets:
Derivatives	0	0
Liabilities:
Derivatives	$ 0	$ 0