Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 125	$ 573
Equity mutual funds	4	0
Fair Value	154	142
Derivative assets	4	5
Derivative liabilities	(26)	0
Corporate debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	93	87
U.S. government and agency debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	22	12
Asset-backed debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	17	20
Certificates of deposit
Assets and liabilities measured at fair value on a recurring basis
Fair Value	18	17
Municipal debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	4	6
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	93	549
Equity mutual funds	4	0
Derivative assets	0	0
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government and agency debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Certificates of deposit
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Significant Other Observable Inputs (Level 2)
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	32	24
Equity mutual funds	0	0
Derivative assets	4	5
Derivative liabilities	(26)	0
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	93	87
Significant Other Observable Inputs (Level 2) | U.S. government and agency debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	22	12
Significant Other Observable Inputs (Level 2) | Asset-backed debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	17	20
Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets and liabilities measured at fair value on a recurring basis
Fair Value	18	17
Significant Other Observable Inputs (Level 2) | Municipal debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	4	6
Significant Unobservable Inputs (Level 3)
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0	0
Equity mutual funds	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Corporate debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | U.S. government and agency debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Asset-backed debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets and liabilities measured at fair value on a recurring basis
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Municipal debt securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	$ 0	$ 0