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Assumptions Used to Estimate Weighted Average Fair Value of Options (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average volatility	50.00%	49.00%	48.00%
Expected dividend yield	2.20%	1.90%	3.00%
Expected term (in years)	4 years 7 months 6 days	4 years 8 months 12 days	5 years 1 month 6 days
Weighted average risk free interest rate	0.70%	1.40%	2.50%

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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