Assumptions Used to Determine Fair Value for Options Granted (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]
Dividend yield		2.152%
Dividend yield, minimum	3.13%		1.68%
Dividend yield, maximum	3.26%		2.37%
Expected volatility, minimum	32.86%	31.57%	32.45%
Expected volatility, maximum	35.27%	33.30%	39.18%
Weighted average expected volatility	34.05%	32.42%	33.50%
Risk-free interest rate, minimum	0.61%	2.00%	0.87%
Risk-free interest rate, maximum	1.60%	2.35%	2.70%
Minimum
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]
Expected term, in years	4 years 6 months	4 years 6 months	2 years 6 months
Maximum
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]
Expected term, in years	5 years 6 months	5 years 6 months	5 years 6 months