Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Options, Outstanding, Beginning Balance	4,220,417
Granted	727,129
Forfeited	(38,874)
Expired	(152,221)
Options, Outstanding, Ending Balance	4,756,451		4,220,417
Exercisable, Ending Balance	2,541,439
Weighted Average Exercise Price, Beginning Balance	$ 13.08
Granted, Weighted Average Exercise Price	1.82	$ 3.02
Forfeited, Weighted Average Exercise Price	5.67
Expired, Weighted Average Exercise Price	19.83
Weighted Average Exercise Price, Ending Balance	11.20		$ 13.08
Exercisable, Weighted Average Exercise Price	$ 17.18
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
Fair value of options, vested	$ 1.5
Unrecognized compensation cost related to nonvested stock-based compensation	$ 4.5
Weighted average remaining period over which unrecognized compensation expense will be recognized	2 years 3 months 18 days
Restricted stock units
Unrecognized compensation cost related to nonvested stock-based compensation	$ 0.7
Weighted average remaining period over which unrecognized compensation expense will be recognized	3 years

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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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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