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Weighted Average Assumptions used in Determining Fair Value for Options Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Expected dividend yield	1.66%	1.53%	1.58%
Expected stock volatility	44.00%	41.00%	44.00%
Risk-free interest rate	1.55%	3.33%	2.73%
Expected holding period	7 years 4 months 24 days	7 years 7 months 6 days	5 years 10 months 24 days
Grant date fair value	$ 15.19	$ 12.49	$ 10.38

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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