Share-based Compensation - Summary of Option Activity (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of options, Outstanding, Beginning Balance	6,548,377	7,228,141	4,309,232
Number of options, Granted	2,759,750	264,480	2,918,993
Number of options, Exercised	(256,065)	(100,834)
Number of options, Forfeited	(290,327)	(843,410)	(84)
Number of options, Outstanding, Ending Balance	8,761,735	6,548,377	7,228,141	4,309,232
Number of options, Vested and Exercisable as of December 31, 2018	3,321,376
Number of options, Vested or expected to vest as of December 31, 2018	8,761,735
Weighted average exercise price, Outstanding, Beginning Balance	$ 1.28	$ 0.97	$ 0.60
Weighted average exercise price, Granted	21.15	8.76	1.53
Weighted average exercise price, Exercised	0.76	0.65
Weighted average exercise price, Forfeited	3.73	1.11	1.74
Weighted average exercise price, Outstanding, Ending Balance	7.47	$ 1.28	$ 0.97	$ 0.60
Weighted average exercise price, Vested and Exercisable as of December 31, 2018	1.01
Weighted average exercise price, Vested or expected to vest as of December 31, 2018	$ 7.47
Weighted average remaining contractual term, Outstanding	7 years 9 months 18 days	8 years 21 days	9 years	9 years 8 months 4 days
Weighted average remaining contractual term, Vested and Exercisable as of December 31, 2018	6 years 11 months 4 days
Weighted average remaining contractual term, Vested or expected to vest as of December 31, 2018	7 years 9 months 18 days
Aggregate intrinsic value, Outstanding	$ 138,009,758	$ 130,668,851	$ 53,677,170	$ 18,874,438
Aggregate intrinsic value, Vested and Exercisable as of December 31, 2018	73,775,871
Aggregate intrinsic value, Vested or expected to vest as of December 31, 2018	$ 138,009,758

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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