Weighted Average Assumptions used to Estimate Fair Value of Stock Options (Detail)	12 Months Ended
Dec. 31, 2011 Times	Dec. 25, 2010 Times	Dec. 26, 2009 Times
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	34.60%	34.30%	34.30%
Risk-free interest rate	3.30%	3.65%	3.00%
Expected dividends	0.00%	0.00%	0.00%
Exercise factor	2.2	2.0	1.7
Discount for post-vesting restrictions	1.10%	1.70%	5.40%

Restrictions on equity-based instruments during the vesting period, such as the inability to transfer unvested awards, are not taken into account in estimating the fair value of the award. However, restrictions that remain in effect after an award is vested, such as the inability to transfer or hedge vested options or a prohibition on the sale of outstanding vested shares (or other type of equity) for a period of time, affect the estimate of an award's fair value.

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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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