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Stock-Based Compensation - Simulation Model Input Variables (Details) (Performance Shares Awards [Member])	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Performance Shares Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility factor	41.00%	39.00%
Risk free interest rate	0.31%	0.45%
Expected term (in years)	2 years 8 months 19 days	2 years 9 months
Expected dividend yield	0.00%	0.00%

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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