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Stock-based Compensation (Details 4) - Stock options
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants    
Risk-free interest rate (as a percent) 1.60% 1.80%
Volatility (as a percent) 77.00% 77.00%
Expected term 6 years 3 months 6 years 3 months