Property and Equipment, Net - Components of Property and Equipment (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 150		$ 122
Less: accumulated depreciation	(58)		(48)
Construction in progress	22	[1]	8	[1]
Property and equipment, net	114		82
Capitalized Software and Website Development
Property Plant And Equipment [Line Items]
Property and equipment, gross	86		73
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	34		22
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	24		21
Furniture, Office Equipment and Other
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 6		$ 6

[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 of our future corporate headquarters in Needham, MA. During the three months ended March 31, 2014 we capitalized $14 million in construction costs with a corresponding liability recorded in other long-term liabilities on our unaudited consolidated balance sheet. 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, 2013 for additional information on our future corporate headquarters lease