Postretirement Benefits Other Than Pensions - Additional Information (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 30, 2012	Dec. 25, 2011	Dec. 26, 2010
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Net actuarial losses recognized in accumulated other comprehensive loss	$ (23.5)	$ (30.8)
Prior service credits recognized in accumulated other comprehensive loss	24.9	44.1
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2012	1.9
Prior service credit estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2012	(9.2)
Annual rate of increase in the per capita cost of covered health care benefits assumed for 2012	6.50%	6.50%	6.50%
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	7.0
Effect on the aggregate service and interest components of the 2011 expense of a 1% change in the health care cost trend rate	$ 0.3