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Fair Value of Each Option Grant Weighted Average Assumptions (Detail)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Dividend yield	0.21%	0.17%
Risk-free interest rate	1.38%	1.35%
Stock price volatility	43.80%	45.00%
Expected life (years)	5 years	5 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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