Stock Based Compensation -Valuation Assumptions Employees (Details) - Employee stock options - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted-average assumptions used to estimate fair value of share-based awards
Risk-free interest rate	1.90%	2.80%	2.10%
Expected volatility	82.10%	82.70%	91.40%
Expected term (in years)	6 years	6 years	6 years
Expected dividend yield	0.00%	0.00%	0.00%
Unrecognized compensation expense	$ 13.4	$ 15.8	$ 18.6
Expected period for recognizing compensation expense	2 years 8 months 16 days	2 years 8 months 19 days	2 years 10 months 6 days

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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- Definition

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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- References

No definition available.

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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 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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- Definition

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

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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