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Stock-Based Compensation Weighted Average Assumptions (Details)	3 Months Ended
	Mar. 31, 2013
Stock-Based Comp [Abstract]
Expected term (years)	8 years 9 months 29 days
Expected volatility	31.87%
Expected dividends	0.00%
Risk-free rate	1.54%

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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