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Assumptions of Weighted Average for ESPP Purchases Utilizing Black-Scholes Model (Detail) (Employee Stock Purchase Plan, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Stock Purchase Plan
Share Based Compensation [Line Items]
Dividend yield	0.90%	0.80%	0.90%
Risk-free interest rate	0.11%	0.16%	0.20%
Expected life in years	6 months	6 months	6 months
Expected volatility	26.00%	17.00%	20.00%
Fair value	$ 21.21	$ 19.63	$ 15.70

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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