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Schedule of Weighted Average Valuation Assumptions (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Risk-free interest rate	0.60%	1.40%	0.70%	1.90%
Dividend yield	0.30%	0.20%	0.30%	0.30%
Volatility factor	43.00%	45.00%	47.00%	44.00%
Expected life (in years)	3 years 6 months	3 years 9 months 18 days	4 years 1 month 6 days	4 years 3 months 18 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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