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Stock-Based Compensation Stock Option Black Scholes Option Pricing Model Assumptions (Details) (Employee Stock Option [Member])	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Employee Stock Option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Dividend yield	0.00%	0.00%
Expected volatility	38.10%	42.70%
Risk-free interest rate	1.00%	0.90%
Expected term (years)	5 years 5 months 11 days	5 years 1 month 21 days

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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