Valuation of debt and equity investments and certain liabilities - Fair value assets and liabilities measured on recurring basis (Details) (Fair Value, Measurements, Recurring, USD $)	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 3,139,000,000	$ 3,791,000,000
Liabilities, Fair Value Disclosure, Recurring	196,000,000	197,000,000
Deferred compensation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Recurring	196,000,000	197,000,000
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	364,000,000	500,000,000
Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	435,000,000	340,000,000
U.S. Government agency and Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	2,160,000,000	2,772,000,000
Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	180,000,000	179,000,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	2,139,000,000	2,786,000,000
Liabilities, Fair Value Disclosure, Recurring	196,000,000	197,000,000
Level 1 | Deferred compensation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Recurring	196,000,000	197,000,000
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	364,000,000	500,000,000
Level 1 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0	0
Level 1 | U.S. Government agency and Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,595,000,000	2,107,000,000
Level 1 | Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	180,000,000	179,000,000
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,000,000,000	1,005,000,000
Liabilities, Fair Value Disclosure, Recurring	0	0
Level 2 | Deferred compensation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value Disclosure, Recurring	0	0
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0	0
Level 2 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	435,000,000	340,000,000
Level 2 | U.S. Government agency and Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	565,000,000	665,000,000
Level 2 | Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0	0
Liabilities, Fair Value Disclosure, Recurring	$ 0	$ 0