Assumptions Used in Black-Scholes Model (Detail)	3 Months Ended
Mar. 31, 2012 Year	Mar. 31, 2011 Year
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Expected term (in years)	6.00	5.75
Expected volatility	27.87%	26.88%
Expected dividend yield	0.00%	0.00%
Risk free interest rate	1.076%	2.28%

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.

+ References+ Details

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.

+ References+ Details