Assumptions Used to Estimate Fair Value of Stock Options Granted (Detail) (USD $)	12 Months Ended
Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Dividend yield	1.80%	0.50%	0.60%
Expected volatility	32.70%	29.90%	32.30%
Risk-free interest rate	1.20%	1.60%	2.00%
Expected term (in years)	4.8	4.6	4.6
Weighted-average fair value of stock options granted	$ 7.85	$ 8.28	$ 7.90

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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