Casualty, Environmental and Other Reserves (Environmental and Other) (Details) $ in Millions	Dec. 25, 2015 USD ($) site	Dec. 26, 2014 USD ($)	Dec. 27, 2013 USD ($)	Dec. 28, 2012 USD ($)
Environmental Reserves [Member]
Contingencies [Line Items]
Reserves	$ 82	$ 94	$ 100	$ 88
Environmental impaired site | site	242
Other Reserves [Member]
Contingencies [Line Items]
Reserves	$ 49	$ 59	$ 71	$ 64