Fair Value Measurement - Assets Measured at Fair Value (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | $ 31,874 | $ 33,632 | |
U.S. government securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 13,088 | 12,766 | |
U.S. government agency securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 9,855 | 10,944 | |
Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 8,931 | 9,922 | |
Fair Value, Measurements, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Total cash equivalents and marketable securities | 41,228 | 39,499 | |
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Total cash equivalents and marketable securities | 31,444 | 29,069 | |
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Total cash equivalents and marketable securities | 9,784 | 10,430 | |
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Total cash equivalents and marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 13,088 | 12,766 | |
Fair Value, Measurements, Recurring | U.S. government securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 13,088 | 12,766 | |
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 9,855 | 10,944 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 9,855 | 10,944 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 8,931 | 9,922 | |
Fair Value, Measurements, Recurring | Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 8,931 | 9,922 | |
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Marketable securities | 0 | 0 | |
Fair Value, Measurements, Recurring | Money market funds | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 6,937 | 5,268 | |
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 6,937 | 5,268 | |
Fair Value, Measurements, Recurring | Money market funds | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | Money market funds | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 1,385 | 66 | |
Fair Value, Measurements, Recurring | U.S. government securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 1,385 | 66 | |
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 179 | 25 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 179 | 25 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | Certificate of deposits and time deposits | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 853 | 440 | |
Fair Value, Measurements, Recurring | Certificate of deposits and time deposits | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 0 | 0 | |
Fair Value, Measurements, Recurring | Certificate of deposits and time deposits | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | 853 | 440 | |
Fair Value, Measurements, Recurring | Certificate of deposits and time deposits | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | $ 0 | 0 | |
Fair Value, Measurements, Recurring | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | | | $ 68 |
Fair Value, Measurements, Recurring | Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | | 0 | |
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Observable Inputs (Level 2) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | | 68 | |
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Unobservable Inputs (Level 3) | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | | |
Cash equivalents | | $ 0 | |