Balance Sheet Components - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Depreciation expense	$ 600,000	$ 400,000	$ 1,500,000	$ 1,100,000
Internal-use software and platform development costs capitalized	800,000	200,000	2,700,000	400,000
Amortization expense related to the internal-use software and platform development costs		0		0
Underlying assets	600,000		600,000
General and Administrative
Property Plant And Equipment [Line Items]
Amortization expense of intangible assets	$ 700,000	$ 700,000	$ 2,000,000	$ 2,100,000
User Relationships
Property Plant And Equipment [Line Items]
Remaining useful life				2 years 6 months	3 years 3 months 18 days