Common Stock (Details) (USD $)	12 Months Ended
Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Stock Options Granted
Weighted average grant date fair value, per share	$ 26.13	$ 11.76	$ 1.34
Weighted average assumptions used:
Expected volatility	70.10%	75.40%	82.60%
Expected lives	4.8	4.6	4.5
Risk-free interest rates	1.80%	2.20%	1.48%
Dividends yields	0.00%	0.00%	0.00%

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.

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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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