Weighted Average Assumptions and Fair Values Used for ESPP (Detail) (Employee Stock Purchase Plan, USD $)	12 Months Ended
Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year
Employee Stock Purchase Plan
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Weighted average expected term of options in years	0.5	0.5	0.5
Weighted average expected volatility of the Company's underlying common stock	27.30%	36.70%	90.00%
Risk-free interest rate	0.15%	0.22%	0.30%
Expected dividend yield
Weighted average fair value	$ 13.02	$ 10.22	$ 10.53

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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