Share-Based Payments - Activity (Details) - Stock option and Stock Appreciation Rights - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Number of Options and SAR
Outstanding at the beginning of the period (in shares)	3,916,963
Granted (in shares)	840,435
Exercised (in shares)	(83,503)
Forfeited (in shares)	(28,024)
Outstanding at the end of the period (in shares)	4,645,871	3,916,963
Vested and expected to vest (in shares)	4,645,871	3,916,963
Exercisable (in shares)	2,593,729	1,889,947
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 19.98
Granted (in dollars per share)	36.76
Exercised (in dollars per share)	12.53
Forfeited (in dollars per share)	36.13
Outstanding at the end of the period (in dollars per share)	23.05	$ 19.98
Vested and expected to vest (in dollars per share)	23.05	19.98
Exercisable (in dollars per share)	$ 15.48	$ 12.47
Weighted-Average Remaining Contractual Term (in years)
Outstanding at the end of the period	7 years 1 month 24 days	7 years 1 month 6 days
Vested and expected to vest	7 years 1 month 24 days	7 years 1 month 6 days
Exercisable	5 years 11 months 23 days	5 years 11 months 15 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 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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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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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 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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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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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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