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Stock-Based Compensation Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected term (years)	6 years 3 months 0 days	6 years 2 months 23 days	6 years 1 month 17 days
Expected volatility	34.14%	34.34%	33.95%
Expected dividends	0.00%	0.00%	0.00%
Risk-free rate	1.22%	0.91%	2.22%

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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