Description of Business and Accounting Policies - Property and Equipment, Net (Details)	12 Months Ended
	Dec. 31, 2017
Building
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	40 years
Internal-use Software and Website Development
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	2 years
Servers
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	3 years
Networking Equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	5 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	5 years
Heavy Equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	10 years