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Stock Based Compensation Plans (Details) (USD $)	12 Months Ended
Stock Based Compensation Plans (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assumptions:
Weighted average risk-free interest rate	0.99%	2.11%	2.22%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	62.70%	59.37%	56.43%
Expected life of options	6 years	5 years 6 months	5 years 4 months 24 days
Weighted average fair value per share at date of grant	$ 17.28	$ 14.31	$ 8.19

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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