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Fair Value of Each Purchase Right Under the ESPP was Estimated on the Date of Grant Using the Black-Scholes Option Valuation Model and the Straight-line Attribution (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years	5 years 1 month 6 days	5 years	5 years 1 month 6 days
Expected volatility	44.00%	45.00%	44.00%	45.00%
Risk-free interest rate	0.84%	0.83%	0.85%	0.82%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months	6 months	6 months	6 months
Expected volatility	27.00%	45.00%	27.00%	45.00%
Risk-free interest rate	0.13%	0.13%	0.13%	0.13%
Expected dividend yield	0.00%	0.00%	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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