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Share Based Compensation - Schedule of Weighted-Average Assumptions (Detail) (Stock Options [Member])	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.55%	0.64%
Dividend yield
Expected volatility	35.05%	34.31%
Expected life (in years)	4 years	4 years

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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