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Weighted-Average Assumptions used to Estimate Fair Values of Stock Options Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free interest rate	1.00%	2.30%	1.90%
Expected volatility	29.00%	33.00%	35.00%
Expected life (in years)	5 years 2 months 12 days	5 years 10 months 24 days	5 years 4 months 24 days
Dividend yield	0.00%	0.00%	0.00%
Weighted-average estimated fair value of options granted during the period	$ 194.27	$ 210.07	$ 216.43

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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