Description of Business and Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Computer hardware and software | Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives	3 years
Computer hardware and software | Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives	5 years
Internal-use software development costs | Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives	3 years
Internal-use software development costs | Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives	4 years
Furniture and fixtures | Minimum
Property Plant And Equipment [Line Items]
Estimated useful lives	1 year
Furniture and fixtures | Maximum
Property Plant And Equipment [Line Items]
Estimated useful lives	5 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated useful lives	Shorter of the estimated lease term or useful life