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Stockholders' Equity (Details)	3 Months Ended		9 Months Ended
Stockholders' Equity (Details)	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Estimated fair value of purchase rights under the Employee stock purchase plan using Black-Scholes valuation model
Expected term (in years)	5 years	5 years 1 month 6 days	5 years 1 month 6 days	5 years 3 months 18 days
Expected volatility	46.00%	45.00%	46.00%	51.00%
Risk-free interest rate	0.67%	0.90%	0.78%	1.65%
Expected dividend yield

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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