Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Subsidiary, Sale of Stock [Line Items]
Restricted cash	$ 2,700,000	$ 2,300,000
Restricted cash, current	2,200,000	1,800,000
Restricted cash, noncurrent	500,000	500,000
Escrow funds payable	98,186,000	87,195,000
Revenue	253,354,000	202,552,000	$ 164,445,000
Gross trade receivables	9,100,000	8,600,000
Gross client receivables	$ 16,000,000	23,800,000
Capitalized costs, amortization period	2 years
Capitalized costs	$ 4,000,000	500,000	0
Internal-use software and platform development costs capitalized	600,000
Internal-use software and platform development costs capitalized, amortization expense	$ 100,000	0	1,000,000
Number of reportable segments | segment	1
Goodwill impairment	$ 0
Capitalized deferred offering costs		100,000
Deferred offering costs		6,300,000
Advertising expense	23,600,000	14,600,000	10,800,000
Foreign currency transactions gain (loss)	$ (400,000)		(500,000)
Minimum
Subsidiary, Sale of Stock [Line Items]
Property and equipment, useful lives	3 years
Intangible asset, useful life	2 years
Maximum
Subsidiary, Sale of Stock [Line Items]
Property and equipment, useful lives	5 years
Intangible asset, useful life	7 years
Foreign Currency Forward Contracts
Subsidiary, Sale of Stock [Line Items]
Notional amount on derivatives	$ 4,800,000	3,600,000
Loss on derivatives	400,000		$ 500,000
Client Accounting For More than 10% revenue
Subsidiary, Sale of Stock [Line Items]
Revenue	$ 29,500,000	$ 24,500,000