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Share Based Compensation (Details) (Employee Stock Option [Member], USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	59.03%	55.88%	46.90%
Risk-free rate	1.24%	2.85%	2.97%
Expected term (in years)	6 years 3 months 2 days	6 years 3 months	6 years 3 months
Expected dividends	$ 0	$ 0	$ 0

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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