Fair Value Measurement of Assets and Liabilities - Financial Assets and Liabilities Measured at Fair Value, Recurring (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Derivatives	$ 105
Liabilities:
Derivatives	0
Recurring
Assets:
Cash and cash equivalents	1,619	$ 2,120
Derivatives		28
Total financial assets	8,857	11,337
Recurring | Short-term Investments
Assets:
Investments, Fair Value Disclosure	2,388	3,743
Recurring | Other Current Assets
Assets:
Derivatives	300
Recurring | Long-term Investments
Assets:
Investments, Fair Value Disclosure	4,550	5,446
Recurring | Other Current Liabilities
Liabilities:
Derivatives	82	29
Recurring | Restricted cash | Short-term Investments
Assets:
Investments, Fair Value Disclosure	17	20
Recurring | Corporate debt securities | Short-term Investments
Assets:
Investments, Fair Value Disclosure	2,371	3,723
Recurring | Corporate debt securities | Long-term Investments
Assets:
Investments, Fair Value Disclosure	4,550	5,446
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	1,619	2,120
Derivatives		0
Total financial assets	1,636	2,140
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term Investments
Assets:
Investments, Fair Value Disclosure	17	20
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Current Assets
Assets:
Derivatives	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term Investments
Assets:
Investments, Fair Value Disclosure	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Current Liabilities
Liabilities:
Derivatives	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Restricted cash | Short-term Investments
Assets:
Investments, Fair Value Disclosure	17	20
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities | Short-term Investments
Assets:
Investments, Fair Value Disclosure	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities | Long-term Investments
Assets:
Investments, Fair Value Disclosure	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Derivatives		28
Total financial assets	7,071	9,197
Recurring | Significant Other Observable Inputs (Level 2) | Short-term Investments
Assets:
Investments, Fair Value Disclosure	2,371	3,723
Recurring | Significant Other Observable Inputs (Level 2) | Other Current Assets
Assets:
Derivatives	150
Recurring | Significant Other Observable Inputs (Level 2) | Long-term Investments
Assets:
Investments, Fair Value Disclosure	4,550	5,446
Recurring | Significant Other Observable Inputs (Level 2) | Other Current Liabilities
Liabilities:
Derivatives	82	29
Recurring | Significant Other Observable Inputs (Level 2) | Restricted cash | Short-term Investments
Assets:
Investments, Fair Value Disclosure	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities | Short-term Investments
Assets:
Investments, Fair Value Disclosure	2,371	3,723
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities | Long-term Investments
Assets:
Investments, Fair Value Disclosure	4,550	$ 5,446
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents	0
Total financial assets	150
Recurring | Significant Unobservable Inputs (Level 3) | Short-term Investments
Assets:
Investments, Fair Value Disclosure	0
Recurring | Significant Unobservable Inputs (Level 3) | Other Current Assets
Assets:
Derivatives	150
Recurring | Significant Unobservable Inputs (Level 3) | Long-term Investments
Assets:
Investments, Fair Value Disclosure	0
Recurring | Significant Unobservable Inputs (Level 3) | Other Current Liabilities
Liabilities:
Derivatives	0
Recurring | Significant Unobservable Inputs (Level 3) | Restricted cash | Short-term Investments
Assets:
Investments, Fair Value Disclosure	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities | Short-term Investments
Assets:
Investments, Fair Value Disclosure	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities | Long-term Investments
Assets:
Investments, Fair Value Disclosure	$ 0