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Share-Based Compensation - Fair Value of Options Granted Determined by using Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk free interest rate	1.27%
Dividend yield	3.38%
Volatility	33.00%

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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