Property and Equipment, Net - Components of Property and Equipment (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 186		$ 122
Less: accumulated depreciation	(77)		(48)
Construction in progress	86	[1]	8	[1]
Property and equipment, net	195		82
Capitalized Software and Website Development
Property Plant And Equipment [Line Items]
Property and equipment, gross	104		73
Property and equipment, net	61		46
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	40		22
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	31		21
Furniture, Office Equipment and Other
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 11		$ 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 and the Company related to our future corporate headquarters in Needham, MA. During the years ended December 31, 2014 and 2013, we capitalized $52 million and $8 million, respectively, 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 $26 million in normal and structural tenant improvements on our consolidated balance sheet incurred by the Company. Refer to “Note 12 – Commitments and Contingencies,” for additional information on our future corporate headquarters lease.