Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-Based Compensation
Options, Outstanding, Beginning Balance	4,220,417
Granted	1,215,382
Exercised	(5,265)
Forfeited	(206,323)
Expired	(609,888)
Options, Outstanding, Ending Balance	4,614,323	4,220,417
Exercisable, Ending Balance	2,490,179
Weighted Average Exercise Price, Beginning Balance	$ 13.08
Granted, Weighted Average Exercise Price	2.08	$ 2.75	$ 12.63
Exercised, Weighted Average Exercise Price	2.84
Forfeited, Weighted Average Exercise Price	6.12
Expired, Weighted Average Exercise Price	18.61
Weighted Average Exercise Price, Ending Balance	9.78	$ 13.08
Exercisable, Weighted Average Exercise Price	$ 15.12
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	6 years 9 months 18 days	6 years 7 months 6 days
Exercisable Ending Balance, Weighted Average Remaining Contractual Term	5 years 2 months 12 days
Outstanding, Intrinsic Value, Ending Balance	$ 2,949
Exercisable, Ending Balance, Intrinsic Value	455
Fair value of options, vested	2,900
Unrecognized compensation cost related to nonvested stock-based compensation	$ 3,500
Weighted average remaining period over which unrecognized compensation expense will be recognized	2 years 2 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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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 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 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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Number of share options (or share units) exercised during the current period.

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