Stockholders' Equity and Employee Benefit Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended	19 Months Ended
Sep. 30, 2018	Dec. 31, 2017	May 31, 2015
Number of Shares
Outstanding, vested and expected to vest at December 31, 2017	13,809,069
Granted	5,455,835		16,700,000
Exercised	(66,265)
Forfeited	(690,207)
Expired	(139,558)
Outstanding, vested and expected to vest at September 30, 2018	18,368,874	13,809,069
Exercisable at September 30, 2018	9,400,643
Weighted Average Exercise Price
Outstanding, vested and expected to vest at December 31, 2017	$ 14.82
Granted	13.32		$ 42.33
Exercised	6.62
Forfeited	15.23
Expired	13.95
Outstanding, vested and expected to vest at September 30, 2018	14.39	$ 14.82
Exercisable at September 30, 2018	$ 14.19
Additional Disclosures
Outstanding, vested and expected to vest, Aggregate Intrinsic Value	$ 30,217	$ 9,292
Exercisable, Aggregate Intrinsic Value at September 30, 2018	$ 17,425
Outstanding, vested and expected to vest, Weighted Average Remaining Contractual Term	7 years 9 months 18 days	7 years 9 months 18 days
Exercisable, Weighted Average Remaining Contractual Term	6 years 8 months 12 days

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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 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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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

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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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