Stock-Based Compensation (Details 2)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Risk-free interest rate	1.05%	1.25%	1.62%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	3.90	3.50	3.60
Expected volatility	35.00%	37.00%	40.00%

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

+ References+ Details

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.

+ References+ Details