Stock Based Compensation, Share Option Valuation (Details) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Stock Option Activity [Roll Forward]
Outstanding at beginning of period (in shares)	8,615,997
Granted (in shares)	1,175,866
Exercised (in shares)	(62,699)
Forfeited (in shares)	(296,589)
Outstanding at end of period (in shares)	9,432,575		8,615,997
Options vested and exercisable at end of period (in shares)	6,949,270
Options unvested at ending of period (in shares)	2,483,305
Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share)	$ 4.48
Granted (in dollars per share)	15.01
Exercised (in dollars per share)	1.81
forfeited (in dollars per share)	6.50
Outstanding at end of period (in dollars per share)	5.75		$ 4.48
Options vested and exercisable at end of period (in dollars per share)	2.55
Options unvested at ending (in dollars per share)	$ 14.69
Weighted-Average Remaining Contractual Term [Abstract]
Outstanding	7 years 6 months 18 days		7 years 6 months 4 days
Granted	9 years 9 months 14 days
Options vested and exercisable	6 years 11 months 1 day
Options unvested	9 years 2 months 12 days
Aggregate Intrinsic Value [Abstract]
Outstanding at beginning of period	$ 94,474
Exercised	1,389
Outstanding at end of period	92,783		$ 94,474
Options vested and exercisable	88,373
Intrinsic value of options exercised and exercisable	$ 88,373	$ 119,000
Weighted average grant date fair value of shares granted (in dollars per share)	$ 9.90	$ 12.93
Total fair value of options vested	$ 35,700	$ 41,800
Employees and Directors [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.57%	2.60%
Expected term	5 years 9 months 14 days	5 years 9 months 11 days
Expected volatility	74.99%	88.60%
Expected dividend yield	0.00%	0.00%
Exercise price (in dollars per share)	$ 15.01	$ 17.63
Fair value of common stock (in dollars per share)	$ 15.01	$ 17.63
Non-Employees [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.74%
Expected term	10 years
Expected volatility	83.79%
Expected dividend yield	0.00%
Exercise price (in dollars per share)	$ 18.75
Fair value of common stock (in dollars per share)	$ 18.75

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The fair value per share of common stock used in the weighted average fair value assumptions for stock option grants.

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Weighted average remaining contractual term for non-vested 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 non-vested.

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

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Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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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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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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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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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for 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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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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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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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Number of non-vested options outstanding.

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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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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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

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