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Schedule of Weighted Average Valuation Assumptions (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Risk-free interest rate	0.70%	1.90%	2.20%
Dividend yield	0.30%	0.30%	0.50%
Volatility factor	46.00%	44.00%	43.00%
Expected life (in years)	4 years	4 years 3 months 18 days	4 years 8 months 12 days

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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