Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 2,281	$ 1,250
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	69.00%	71.00%
Expected option term	6 years	6 years
Risk-free interest rate (as a percent)	1.80%	2.20%
Expected dividend yield (as a percent)	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	1,269	591
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,012	$ 659

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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

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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.

+ References

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

-Publisher FASB

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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

-Publisher FASB

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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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