Stock-Based Compensation - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense, outstanding stock options \| $	$ 52.9	$ 52.9
Stock options vested and expected to vest (in shares) \| shares	19,400,000	19,400,000
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant (in shares) \| shares	10,905,099	10,905,099
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, stock incentive plans, maximum term		10 years
Estimated remaining weighted average period		3 years 2 months 12 days
Stock options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, stock incentive plan, vesting period		3 years
Stock options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, stock incentive plan, vesting period		4 years
Stock-based awards \| 2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares) \| shares	9,900,000	9,900,000
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated remaining weighted average period		2 years 3 months 18 days
Unrecognized stock-based compensation expense, unvested restricted stock or RSUs \| $	$ 3.8	$ 3.8
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated remaining weighted average period		2 years 3 months 18 days
Unrecognized stock-based compensation expense, unvested restricted stock or RSUs \| $	$ 15.9	$ 15.9
Restricted stock units \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, stock incentive plan, vesting period		2 years
Restricted stock units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, stock incentive plan, vesting period		4 years
Employee stock purchase plan \| 2018 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated remaining weighted average period	1 year 10 months 24 days

Aggregate number of common shares reserved for future issuance.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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