Employee Benefit Plans, Assumptions and Resulting Estimates of Fair Value (Details) (USD $)	12 Months Ended
Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Employee Stock Purchase Plan 2008 [Member]
Estimates of Fair Value
Expected Volatility Rate	41.00%	35.00%	54.00%
Risk Free Interest Rate	0.20%	0.20%	0.40%
Expected Term	0.5	0.5	0.5
Dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per share	$ 7.48	$ 6.55	$ 5.54
Stock Options [Member]
Estimates of Fair Value
Expected Volatility Rate	43.00%	38.00%	50.00%
Risk Free Interest Rate	1.50%	2.00%	1.60%
Expected Term	4	4	4
Dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per share	$ 13.17	$ 9.77	$ 7.41

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.

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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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