Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	$ 9,631	$ 8,126
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	11,507	10,405
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	1,876	2,279
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	6,759	5,687
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	2,872	2,439
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	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | US Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | US Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	0	0
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 | Significant Unobservable Inputs (Level 3) | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | US Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | US Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	0	0
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Total cash equivalents and marketable securities	11,507	10,405
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	1,876	2,279
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure | US Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	6,759	5,687
Fair Value, Measurements, Recurring | Estimate of Fair Value, Fair Value Disclosure | US Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Marketable Securities, Current	$ 2,872	$ 2,439