Summary of Significant Accounting Policies - Additional Information (Details)					3 Months Ended		9 Months Ended
		Sep. 25, 2018 USD ($)	Feb. 16, 2018 USD ($)	Jan. 12, 2017 USD ($)	Sep. 30, 2018 USD ($) Instrument	Sep. 30, 2017 USD ($) Instrument	Sep. 30, 2018 USD ($) Instrument	Sep. 30, 2017 USD ($) Instrument	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue, revenue recognized					$ 43,100,000	$ 36,300,000	$ 78,200,000	$ 68,800,000
Revenue, unsatisfied performance obligation					$ 109,900,000		$ 109,900,000
Performance obligation, expected timing of satisfaction, period					12 months		12 months
Deferred commissions, amortization period					4 years		4 years
Amortization of deferred commissions					$ 423,000	260,000	$ 1,107,000	688,000
Impairment loss related to deferred commissions					0	0	$ 0	0
Stock-based compensation, requisite service period							4 years
Cumulative amount of unrecognized stock-based compensation expense recognized					98,812,000	8,422,000	$ 116,104,000	26,363,000
Impairment of long-lived assets					0	0	0	0
Impairment of goodwill					0	$ 0	0	$ 0
Private company investment					3,600,000		3,600,000				$ 5,000,000
Impairments on private company investment					0		0
Observable price changes for private company investment					$ 0		$ 0
Derivative financial instruments | Instrument					0	0	0	0
Liability for sabbatical leave, accrued balance					$ 4,200,000		$ 4,200,000			$ 3,600,000
Advertising and promotion costs					6,600,000	$ 4,500,000	17,100,000	$ 13,400,000
Cash consideration from sale of private company investment				$ 11,700,000
Gain on sale of private company investment					0	0	$ 999,000	6,750,000
Sale of private company investment, period of contingent consideration to be received							3 years
Earn-out payment received			$ 1,000,000
Sales and marketing expense	[1],[2]				34,309,000	18,878,000	$ 71,609,000	55,791,000
Total assets					790,311,000		790,311,000			578,089,000
Adjustments of retained earnings					(307,070,000)		(307,070,000)			(177,571,000)
Accounting Standards Update 2018-07
Summary Of Significant Accounting Policies [Line Items]
Adjustments of retained earnings									$ (43,000)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Summary Of Significant Accounting Policies [Line Items]
Sales and marketing expense						$ (17,000)		$ (688,000)
Total assets										3,200,000
Accrued Compensation
Summary Of Significant Accounting Policies [Line Items]
Liability for sabbatical leave, accrued balance					2,100,000		2,100,000			1,700,000
Other Non-Current Liabilities
Summary Of Significant Accounting Policies [Line Items]
Liability for sabbatical leave, accrued balance					2,100,000		$ 2,100,000			$ 1,900,000
Capitalized Internal-Use Software
Summary Of Significant Accounting Policies [Line Items]
Intangible assets estimated useful life							3 years
Customer Concentration Risk | Revenue
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, customer							No single customer accounted for more than 10% of revenue
Customer Concentration Risk | Accounts Receivable
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, customer							No customers accounted for more than 10% of accounts receivable, net
Out-of-the-Money Stock Option and At-the-Money Stock Option
Summary Of Significant Accounting Policies [Line Items]
Share-based compensation, fair value assumptions, expected dividend rate							0.00%
Performance Based RSUs
Summary Of Significant Accounting Policies [Line Items]
Cumulative amount of unrecognized stock-based compensation expense recognized		$ 89,900,000
Remaining unamortized stock-based compensation					$ 37,900,000		$ 37,900,000
Remaining weighted-average requisite service periods							1 year 7 months 6 days

[1] Includes amortization of acquisition intangible assets as follows: Three Months Ended September 30, Nine Months Ended September 30, (in thousands) 2018 2017 2018 2017 Cost of revenue $488 $488 $1,464 $1,552 Sales and marketing 565 604 1,773 1,817 Amortization of acquisition intangible assets $1,053 $1,092 $3,237 $3,369

[2] Includes stock-based compensation, net of amounts capitalized as follows: Three Months Ended September 30, Nine Months Ended September 30, (in thousands) 2018 2017 2018 2017 Cost of revenue $6,472 $634 $7,776 $1,870 Research and development 37,490 2,799 43,903 7,065 Sales and marketing 14,496 1,322 16,411 6,622 General and administrative 40,354 3,667 48,014 10,806 Stock-based compensation, net of amounts capitalized $98,812 $8,422 $116,104 $26,363