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Share-Based Compensation - Assumptions Used in Black Scholes Model (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeiture rate	5.00%
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.36%	2.74%	2.38%
Expected price volatility	46.22%	45.43%	43.54%
Dividend yield	3.29%	3.65%	4.02%
Forfeiture rate	4.77%	5.00%	5.00%
Weighted-average expected life, until exercise	7 years 2 months 12 days	6 years 10 months 28 days	6 years 8 months 5 days

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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