Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Stockholders' Equity
Options, Outstanding, Beginning Balance	2,675,184
Granted	569,199
Exercised	(858)
Forfeited	(162,420)
Expired	(126,030)
Options, Outstanding, Ending Balance	2,955,075		2,675,184
Exercisable, Ending Balance	1,901,703
Weighted Average Exercise Price, Beginning Balance	$ 23.52
Granted, Weighted Average Exercise Price	17.42	$ 12.80
Exercised, Weighted Average Exercise Price	12.77
Forfeited, Weighted Average Exercise Price	19.04
Expired, Weighted Average Exercise Price	55.07
Weighted Average Exercise Price, Ending Balance	21.25		$ 23.52
Exercisable, Weighted Average Exercise Price	$ 22.98
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	6 years 7 months 6 days		6 years 6 months
Exercisable Ending Balance, Weighted Average Remaining Contractual Term	5 years 4 months 24 days
Outstanding, Intrinsic Value, Ending Balance	$ 1,191		$ 5,805
Exercisable, Ending Balance, Intrinsic Value	1,190
Fair value of options, vested	3,800
Unrecognized compensation cost related to nonvested stock-based compensation	$ 9,600
Weighted average remaining period over which unrecognized compensation expense will be recognized	2 years 4 months 24 days

Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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 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 per share amount at which grantees can acquire shares of common stock by exercise of options.

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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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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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