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Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail) (ESPP)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.13%	0.16%
Expected life (months)	6 months	8 months 15 days
Expected volatility	53.14%	53.00%
Expected dividend yield	0.00%	0.00%

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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