Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding, beginning (shares)	13,857,213	14,340,582	13,836,851
Granted (shares)	1,094,358	1,223,483	1,435,550
Exercised (shares)	(7,858,141)	(937,482)	(39,010)
Forfeited (shares)	(214,464)	(244,793)	(379,268)
Expired (shares)	(35,381)	(524,577)	(513,541)
Outstanding, ending (shares)	6,843,585	13,857,213	14,340,582
Vested (exercisable) (shares)	4,955,073
Unvested (unexercisable) (shares)	1,888,512
Weighted- Average Exercise Price/Share
Outstanding, beginning ($ per share)	$ 6.89	$ 6.86	$ 6.97
Granted ($ per share)	18.87	6.51	5.94
Exercised ($ per share)	6.74	6.40	5.18
Forfeited ($ per share)	6.54	6.26	7.21
Expired ($ per share)	6.94	6.27	7.26
Outstanding, ending ($ per share)	8.98	$ 6.89	$ 6.86
Vested (exercisable) ($ per share)	7.87
Unvested (unexercisable) ($ per share)	$ 11.91
Weighted-Average Remaining Contractual Term (years), outstanding	6 years 4 months 20 days
Weighted-Average Remaining Contractual Term (years), vested (exercisable)	5 years 7 months 13 days
Weighted-Average Remaining Contractual Term (years), unvested (unexercisable)	8 years 4 months 17 days
Exercised, Aggregate Intrinsic Value	$ 84,758	$ 6,813	$ 50
Outstanding, Aggregate Intrinsic Value	11,095
Aggregate Intrinsic Value, vested (exercisable)	8,422
Aggregate Intrinsic Value, unvested (unexercisable)	$ 2,673

Share Based Compensation Arrangement By Share Based Payment Award Options Unvested In Period

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Share Based Compensation Arrangement By Share Based Payment Award Options Unvested Intrinsic Value

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Share Based Compensation Arrangement By Share Based Payment Award Options Unvested Weighted Average Exercise Price

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Share Based Compensation Arrangement By Share Based Payment Award Options Unvested Weighted Average Remaining Contractual Term Ending Balance

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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 vested portions of options outstanding and currently exercisable or convertible, 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 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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Number of share options (or share units) exercised during the current period.

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