Stock-Based Compensation - Fair Market Value of Stock Option Awards (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation
Aggregate fair value of stock option awards vested	$ 644	$ 1,418	$ 1,869
Aggregate intrinsic value of outstanding stock option awards	5,503	1,969	1,100
Aggregate intrinsic value of stock option awards exercised	$ 748	$ 155	$ 311
Weighted-average grant-date fair value per share of stock option awards granted (usd per share)	$ 1.13	$ 0.57	$ 0.80
Cash proceeds received from stock option awards exercised	$ 699	$ 161	$ 108
Stock option awards
Stock-Based Compensation
Unrecognized compensation cost related to non-vested stock option awards	$ 1,042
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 8 months 12 days
Assumptions used to compute the fair value of stock option awards
Dividend yield (percent)	0.00%	0.00%	0.00%
Stock option awards \| Minimum
Assumptions used to compute the fair value of stock option awards
Volatility (percent)	49.00%	32.00%	34.00%
Risk-free interest rate (percent)	1.85%	1.24%	1.60%
Expected life (in years)	5 years 7 months 23 days	5 years 9 months 18 days	6 years
Stock option awards \| Maximum
Assumptions used to compute the fair value of stock option awards
Volatility (percent)	50.00%	33.00%	43.00%
Risk-free interest rate (percent)	2.02%	1.57%	1.73%
Expected life (in years)	5 years 7 months 30 days	6 years 11 days	6 years 22 days

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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Unrecognized cost of unvested options awarded to employees as compensation.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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 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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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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