Casualty, Environmental and Other Reserves (Casualty, Environmental and Other Reserves Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 25, 2015	Dec. 26, 2014		Dec. 27, 2013
Casualty Reserves [Member]
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 265	$ 280		$ 325
Charged to Expense	60	89	[1]	54
Payments	(56)	(104)		(99)
Balance end of period	269	265		280
Environmental Reserves [Member]
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	94	100		88
Charged to Expense	45	57	[1]	48
Payments	(57)	(63)		(36)
Balance end of period	82	94		100
Other Reserves [Member]
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	59	71		64
Charged to Expense	37	30	[1]	38
Payments	(47)	(42)		(31)
Balance end of period	49	59		71
Casualty, Environmental and Other [Member]
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	418	451		477
Charged to Expense	142	176	[1]	140
Payments	(160)	(209)		(166)
Balance end of period	$ 400	$ 418		$ 451

[1] Increase in expense in 2014 is primarily due to the resolution of personal injury claims for prior years.