Property and Equipment, Net - Components of Property and Equipment (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 207		$ 186
Less: accumulated depreciation	(91)		(77)
Construction in progress	110	[1]	86	[1]
Property and equipment, net	226		195
Capitalized Software and Website Development
Property Plant And Equipment [Line Items]
Property and equipment, gross	118		104
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	42		40
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	33		31
Furniture, Office Equipment and Other
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 14		$ 11

[1] We capitalize construction in progress for build-to-suit lease agreements where we are considered the owner, for accounting purposes only, during the construction period. These amounts represent construction costs to date incurred by the landlord and the Company related to our future corporate headquarters in Needham, MA. During the three months ended March 31, 2015, we capitalized $4 million in non-cash construction costs which were incurred by the landlord, with a corresponding liability recorded in other long-term liabilities, and in addition, we capitalized $20 million in tenant improvements on our unaudited consolidated balance sheet, of which $13 million were paid in cash by the Company. Refer to “Note 12 – Commitments and Contingencies,” in the notes to our consolidated financial statements in Item 8 of our Annual Report on Form 10-K for the year ended December 31, 2014 for additional information on our future corporate headquarters lease.