Nature of Operations and Significant Accounting Policies - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 27, 2015 mi state	Mar. 28, 2014	Dec. 26, 2014
Background
Total number of rail route miles	21,000
Number of states rail network serves	23
Fiscal Year
Number of weeks in fiscal year	364 days	364 days
Number of weeks in fiscal quarter	91 days	91 days
Number of weeks in fiscal quarter year-to-date	91 days	91 days
Allowance for Doubtful Accounts
Allowance for doubtful accounts	$ 39		$ 41