Equity and Employee Stock Plans - Summary of Common Stock Reserved for Future Issuance (in shares) (Details) - shares	Sep. 30, 2019	Dec. 31, 2018	Jul. 26, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares underlying outstanding stock options (in shares)	8,102,171	9,435,349
Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares underlying outstanding stock options (in shares)	8,102,171	9,435,349
Shares underlying outstanding restricted stock units (in shares)	4,536,677	3,264,702
Shares available for future equity grants (in shares)	7,837,918	5,068,013
Common stock reserved (in shares)	20,476,766	17,768,064	7,898,159
2017 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance on January 1, 2019 and 2018, respectively (in shares)	2,890,973	2,414,688
Shares issued since January 1, 2019 and 2018, respectively (in shares)	(262,110)	(425,228)
Common stock reserved (in shares)	2,628,863	1,989,460

Aggregate number of common shares reserved for future issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
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The shares that, although not legally released, will be released by a future scheduled and committed debt service payment and will be allocated to employees for service rendered in the current accounting period. The ESOP documents typically define the period of service to which the shares relate. ESOP shares are released to compensate employees directly, to settle employer liabilities for other employee benefits, and to replace dividends on allocated shares that are used for debt service.

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-Publisher FASB
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-Topic 718
-SubTopic 40
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Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of options outstanding, including both vested and non-vested options.

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