Assumptions Used to Determine Compensation Cost for Market Condition Component of Performance Share Program Plan (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2012
Share Based Compensation [Line Items]
Expected volatility, lower limit	28.39%	28.39%
Expected volatility, upper limit	55.33%	55.33%
Risk-free interest rate, lower limit	0.12%	0.12%
Risk-free interest rate, upper limit	0.42%	0.42%

The estimated measure of the maximum 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.

No definition available.

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