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Stock Based Compensation (Details)	12 Months Ended
Stock Based Compensation (Details)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Weighted average assumptions relevant to determining the fair value at the dates of grant
Term	6 years	6 years	6 years
Risk free interest rate	1.16%	1.72%	2.37%
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	50.51%	42.10%	63.90%
Expected forfeiture rate	3.00%	5.00%	6.20%

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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