Summary of Significant Accounting Policies - Additional Information (Details)		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 25, 2018 USD ($)	Sep. 30, 2019 USD ($) Instrument	Sep. 30, 2018 USD ($) Instrument	Sep. 30, 2019 USD ($) Instrument	Sep. 30, 2018 USD ($) Instrument	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue, revenue recognized		$ 55,400,000	$ 43,100,000	$ 95,300,000	$ 78,200,000
Deferred commissions, amortization period		4 years		4 years
Amortization of deferred commissions		$ 700,000	400,000	$ 1,900,000	1,100,000
Impairment loss related to deferred commissions		$ 0	$ 0	$ 0	$ 0
Share-based compensation, fair value assumptions, expected dividend rate		0.00%	0.00%	0.00%	0.00%
Cumulative amount of unrecognized stock-based compensation expense recognized	$ 89,900,000	$ 14,782,000	$ 98,812,000	$ 45,047,000	$ 116,104,000
Remaining unamortized stock-based compensation		9,300,000		$ 9,300,000
Remaining weighted-average requisite service periods				1 year 2 months 12 days
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		$ 3,600,000
Impairments on private company investment		0	0	0	0
Observable price changes for private company investment		$ 0	$ 0	$ 0	$ 0
Derivative financial instruments outstanding | Instrument		0	0	0	0
Deferred rent balance						7,500,000
Liability for sabbatical leave, accrued balance		$ 3,700,000		$ 3,700,000		4,400,000
Advertising and promotion costs		6,100,000	$ 6,600,000	$ 18,900,000	$ 17,100,000
Sale of private company investment, period of contingent consideration to be received				3 years
Earn-out payment received				$ 1,000,000	$ 1,000,000
Recognition of ROU assets		65,134,000		65,134,000		0
Operating lease liabilities		92,865,000		92,865,000
Accrued Expenses And Other Current Liabilities
Summary Of Significant Accounting Policies [Line Items]
Deferred rent balance						300,000
Other Non-Current Liabilities
Summary Of Significant Accounting Policies [Line Items]
Deferred rent balance						7,200,000
Liability for sabbatical leave, accrued balance		2,000,000		2,000,000		2,100,000
Accrued Compensation
Summary Of Significant Accounting Policies [Line Items]
Liability for sabbatical leave, accrued balance		$ 1,700,000		$ 1,700,000		$ 2,300,000
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Recognition of ROU assets							$ 63,100,000
Operating lease liabilities							$ 92,800,000
ASC 842 | Minimum
Summary Of Significant Accounting Policies [Line Items]
Operating lease terms		1 year		1 year
ASC 842 | Maximum
Summary Of Significant Accounting Policies [Line Items]
Operating lease terms		12 years		12 years
Capitalized Internal-Use Software
Summary Of Significant Accounting Policies [Line Items]
Intangible assets estimated useful life				3 years
Customer Concentration Risk | Sales Revenue, Net
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, customer		No single customer accounted for more than 10% of revenue	No single customer accounted for more than 10% of revenue	No single customer accounted for more than 10% of revenue	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		No customers accounted for more than 10% of accounts receivable, net		No customers accounted for more than 10% of accounts receivable, net
Equity Incentive Plans | Time-Based Awards | New Hires
Summary Of Significant Accounting Policies [Line Items]
Stock-based compensation, requisite service period				4 years
Equity Incentive Plans | Time-Based Awards | Existing Employees
Summary Of Significant Accounting Policies [Line Items]
Stock-based compensation, requisite service period				3 years