Stock-based Compensation Stock-based Compensation (Details 3)	3 Months Ended	9 Months Ended
Sep. 30, 2012 Years	Sep. 30, 2011 Years	Sep. 30, 2012 Years	Sep. 30, 2011 Years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.00%	0.00%	0.40%	1.00%
Expected dividend yield	0.00%	0.00%	1.30%	0.70%
Expected term	0	0	4.2	4.3
Expected volatility	0.00%	0.00%	39.70%	42.30%
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.00%	0.00%	0.20%	0.10%
Expected dividend yield	0.00%	0.00%	1.30%	0.70%
Expected term	0	0	6	6
Expected volatility	0.00%	0.00%	27.40%	21.30%

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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