Assumptions Used to Value Options Granted (Detail) (USD $)	6 Months Ended
Jun. 30, 2011 Year	Jun. 30, 2010 Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.40%	3.30%
Dividend yield	1.50%	1.60%
Expected volatility	31.70%	30.30%
Expected life (years)	5.4	6.5
Fair value per option	$ 30.87	$ 23.84

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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