Assumptions used in Black-Scholes Option-Pricing Model for Determining Fair Value of Options Granted (Detail) (USD $)	12 Months Ended
Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Stock options granted	1,050,226	1,388,846	1,368,476
Weighted average grant date fair value	$ 21.07	$ 13.74	$ 11.90
Expected term (in years)	5.75	5.75	5.50
Expected volatility	27.42%	28.31%	28.28%
Expected dividend yield	0.00%	0.00%	0.00%
Risk free interest rate	2.21%	2.33%	2.08%

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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