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Pension Expense - Summary of Net Pension and Supplemental Executive Retirement Plan Expense Recognized (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 15, 2013
Jun. 16, 2012
Jun. 15, 2013
Jun. 16, 2012
Compensation And Retirement Disclosure [Abstract]        
Service cost pertaining to benefits earned during the period $ 2.1 $ 1.8 $ 4.2 $ 3.5
Interest cost on projected benefit obligations 4.3 3.3 8.7 6.6
Expected return on pension assets (4.8) (3.4) (9.7) (6.9)
Net amortization loss 7.0 4.8 14.0 9.7
Net pension expense $ 8.6 $ 6.5 $ 17.2 $ 12.9