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Assumptions Used for Calculating Fair Values of Stock Option Awards (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Stock Options Granted
Weighted average grant date fair value, per share	$ 19.84	$ 17.35
Weighted average assumptions used:
Expected volatility	66.40%	73.50%
Expected lives	4 years 10 months 24 days	4 years 8 months 12 days
Risk-free interest rates	0.70%	0.80%
Dividend yields	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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