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Stock Based Compensation - Fair Value of Share Based Payment Awards (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected lives of options, years	7 years 9 months 18 days	4 years 7 months 6 days	7 years 7 months 6 days
Expected volatility	44.60%	47.80%	44.20%
Risk free interest rate	1.70%	1.30%	2.80%
Dividend yield rate	4.00%	4.10%	3.50%

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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