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Stock-Based Compensation (Details) (USD $)	3 Months Ended
Stock-Based Compensation (Details) (USD $)	Mar. 31, 2013	Mar. 31, 2012
Summary of assumptions used to estimate fair value of each SAR
Weighted-average fair value of grants	$ 23.34	$ 14.01
Dividend yield	0.00%	0.00%
Average risk-free interest rate	0.70%	0.80%
Expected term (years)	5 years	5 years
Expected volatility	64.00%	66.00%

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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