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Nature of Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 27, 2014 states mi | Jun. 28, 2013 | Jun. 27, 2014 states mi | Jun. 28, 2013 | Dec. 27, 2013
Background |  |  |  |  |
Total number of rail route miles | 21,000 |  | 21,000 |  |
Rail network states number | 23 |  | 23 |  |
Fiscal Year |  |  |  |  |
Number of weeks in fiscal quarter | 91 days | 91 days |  |  |
Number of weeks in fiscal quarter ytd |  |  | 182 days | 182 days |
Number of weeks in fiscal year |  |  | 364 days | 364 days |
Allowance for Doubtful Accounts |  |  |  |  |
Allowance for doubtful accounts | $ 35 |  | $ 35 |  | $ 33

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