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Employee Benefit Plans Assumptions Used in Estimating the Grant Date Fair Value of Options Using the Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dividend yield	0.91%	0.75%	0.17%
Expected stock price volatility	39.88%	34.87%	56.09%
Risk-free interest rate (weighted average)	1.07%	2.48%	2.80%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	6 years	6 years	6 years

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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