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Share-Based Payments (Details 2) (USD $)	12 Months Ended
Share-Based Payments (Details 2) (USD $)	Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Weighted-average assumptions to estimate the fair value of the stock option grants awarded
Expected option life (in years)	0.0	4.5	4.7
Expected stock price volatility	0.00%	30.80%	39.30%
Risk-free interest rate	0.00%	2.00%	1.90%
Expected dividend yield	0.00%	0.00%	0.00%
Per share grant-date fair value	$ 0.00	$ 16.52	$ 18.00

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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