Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Options, Outstanding, Beginning Balance	3,304,531
Granted	610,002
Forfeited	(60,595)
Expired	(9,561)
Options, Outstanding, Ending Balance	3,844,377		3,304,531
Exercisable, Ending Balance	2,217,289
Weighted Average Exercise Price, Beginning Balance	$ 18.42
Granted, Weighted Average Exercise Price	3.02	$ 17.42
Forfeited, Weighted Average Exercise Price	13.36
Expired, Weighted Average Exercise Price	40.42
Weighted Average Exercise Price, Ending Balance	15.99		$ 18.42
Exercisable, Weighted Average Exercise Price	$ 21.36
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	6 years 7 months 6 days		6 years 7 months 6 days
Exercisable Ending Balance, Weighted Average Remaining Contractual Term	4 years 10 months 24 days
Outstanding, Intrinsic Value, Ending Balance	$ 9
Fair value of options, vested	2,500
Unrecognized compensation cost related to nonvested stock-based compensation	$ 6,200
Weighted average remaining period over which unrecognized compensation expense will be recognized	2 years 6 months
Restricted Stock Activity
Unrecognized compensation cost related to nonvested stock-based compensation	$ 500
Weighted average remaining period over which unrecognized compensation expense will be recognized	3 years 6 months

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for 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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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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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 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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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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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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