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Assumptions Used in Valuation of Stock Options (Detail)	3 Months Ended		9 Months Ended
Assumptions Used in Valuation of Stock Options (Detail)	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Share Based Compensation [Line Items]
Risk-free interest rate	1.00%	2.50%	1.00%	2.10%
Expected life in Years	6 years 2 months 12 days	6 years 3 months 18 days	6 years 3 months 18 days	6 years 2 months 12 days
Expected volatility	20.80%	21.10%	20.70%	20.90%
Dividend Yield	1.70%	1.80%	1.80%	1.70%

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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