Weighted-Average Assumptions (Detail) (USD $)	12 Months Ended
Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Expected dividend yield	3.20%	3.50%	3.20%
Expected stock price volatility	21.50%	22.10%	24.40%
Risk-free interest rate	2.80%	2.80%	2.00%
Expected life of options In years	6.3	6.2	6.2
Fair value per option granted	$ 12.18	$ 9.90	$ 9.66

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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