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Postretirement Benefits Other Than Pensions - Additional Information (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 25, 2011
Dec. 26, 2010
Dec. 27, 2009
Other Postretirement Benefit Plans, Defined Benefit
      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net actuarial losses recognized in accumulated other comprehensive loss $ (30.8) $ (33.7)  
Prior service credits recognized in accumulated other comprehensive loss (44.1) (63.6)  
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2012 5.3    
Prior service credit estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2012 19.2    
Annual rate of increase in the per capita cost of covered health care benefits assumed for 2012 6.50% 6.50% 7.00%
Percentage of change in the health care cost trend rate 1.00%    
Effect on postretirement benefit obligation of a 1% change in the health care cost trend rate 7.5    
Effect on the aggregate service and interest components of the 2011 expense of a 1% change in the health care cost trend rate $ 0.4