Property and Equipment, Net - Components of Property and Equipment, Net (Details) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross		$ 561	$ 493
Less: accumulated depreciation		(296)	(240)
Total		265	253	[1]
Capitalized Software and Website Development
Property Plant And Equipment [Line Items]
Property and equipment, gross		315	259
Building
Property Plant And Equipment [Line Items]
Property and equipment, gross	[2]		123
Finance Lease Right-of-Use Asset
Property Plant And Equipment [Line Items]
Property and equipment, gross	[2]	114
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross		46	41
Computer Equipment and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, gross		66	52
Furniture, Office Equipment and Other
Property Plant And Equipment [Line Items]
Property and equipment, gross		$ 20	$ 18

[1] Refer to the below discussion regarding the transition accounting for operating and finance leases upon adoption of ASC 842.

[2] Refer to “Note 2: Significant Accounting Policies” regarding the transition accounting related to our adoption of ASC 842 and the subsequent accounting for our Headquarters Lease.