Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 708	$ 641
Stock options
Assumptions used for fair values of employee stock options granted
Expected stock price volatility (as a percent)	72.00%	70.00%
Expected option term	6 years	6 years
Risk-free interest rate (as a percent)	1.40%	1.40%
Expected dividend yield (as a percent)	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	441	396
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 267	$ 245

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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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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