Employee benefit plans and stock-based compensation plans reflecting the two-for-one stock split (Details 2)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Weighted average assumptions to value options and SSRs Granted
Expected volatility of stock	36.60%	38.40%	37.50%
Expected dividend yield	0.00%	0.00%	0.00%
Minimum [Member]
Weighted average assumptions to value options and SSRs Granted
Expected life of option, in years	2	3	3
Risk-free interest rate	0.30%	0.50%	1.30%
Expected volatility of stock	30.00%	36.00%	35.00%
Maximum [Member]
Weighted average assumptions to value options and SSRs Granted
Expected life of option, in years	5	5	5
Risk-free interest rate	2.20%	2.40%	2.40%
Expected volatility of stock	39.00%	41.00%	39.00%

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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