Fair Value Measurements (Details 1) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair value measurements using quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	$ 4,496,000,000	$ 3,909,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	4,185,000,000	3,511,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	300,000,000	387,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	11,000,000	11,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	9,000,000	8,000,000
Fair value measurements using significant unobservable inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3) | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	9,000,000	8,000,000
Fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	4,505,000,000	3,917,000,000
Fair value | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	4,185,000,000	3,511,000,000
Fair value | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	300,000,000	387,000,000
Fair value | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	11,000,000	11,000,000
Fair value | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	$ 9,000,000	$ 8,000,000