Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) FinancialInstitution	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Unrealized loss on short-term investments	$ 17,000	$ 0	$ 0
Foreign currency gains (loss)	$ (500,000)	100,000	(400,000)
Number of financial institutions which manage cash balances | FinancialInstitution	1
Impairment of goodwill	$ 0	0
Impairment of long-lived assets	400,000	0
Advertising costs	$ 3,000,000	3,400,000	3,900,000
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	5 years
Property and equipment, estimated useful lives description	shorter of the lease term or their estimated useful lives, which is generally five years
Website and Internal-Use Software
Summary Of Significant Accounting Policies [Line Items]
Capitalized costs	$ 1,000,000	2,600,000	1,800,000
Amortized expenses	$ 2,000,000	$ 1,100,000	$ 700,000
Estimated useful life	2 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Cash equivalents maturity period	90 days
Property and equipment, estimated useful lives	5 years
Maximum | Subsequent Event | ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating leases, right-of-use assets				$ 45,000,000
Operating leases, liabilities				50,000,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	18 months
Minimum | Subsequent Event | ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating leases, right-of-use assets				35,000,000
Operating leases, liabilities				$ 40,000,000
Cash, Cash Equivalents and Short-term Investment
Summary Of Significant Accounting Policies [Line Items]
Percentage of cash, cash equivalents balances	97.00%	95.00%