Stock-Based Compensation (Details 5) (Employee Stock Purchase Plan [Member], USD $)	12 Months Ended
Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Employee Stock Purchase Plan [Member]
Stock compensation relating to Black-Scholes option pricing model
Expected life (years)	1.0	1.0	1.0
Risk-free interest rate	0.10%	0.20%	0.30%
Volatility	36.60%	44.20%	69.50%
Dividend yield	1.00%	1.50%	1.90%
Fair value per share of the 15% cash discount	$ 9.62	$ 6.16	$ 4.81
Fair value per share of the look-back provision	$ 6.50	$ 4.98	$ 8.44
Total weighted average fair value per share at grant date	$ 16.12	$ 11.14	$ 13.25

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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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the expected term of a nonvested share or option award issued to other than an employee.

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- Definition

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 holders' expected behavior.

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- Definition

The risk-free interest rate assumption that is used in valuing a nonvested share or option award issued to other than an employee.

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