Stock Awards - Weighted Average Assumptions - Tabular Disclosure (Details) - Employee Stock Options	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Assumptions used in estimating the fair value of stock options granted
Risk-free interest rate	2.82%	1.93%	2.72%	2.07%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (in years)	6 years	5 years 10 months 24 days	6 years	6 years
Expected stock price volatility	69.82%	74.64%	70.11%	75.23%

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- Definition

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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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Weighted average expected volatility rate of share-based compensation awards.

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- Definition

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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-Name Accounting Standards Codification
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