| **Summary of significant accounting policies (Details)**                                        | Jul. 31, 2012segments |
|-------------------------------------------------------------------------------------------------|-----------------------|
| **Accounting Policies [Line Items]**                                                            |                  |
| Number Of Segments                                                                              | 4                     |
| Customer Concentration Risk [Member]                                                            |                  |
| **Risks and Uncertainties [Abstract]**                                                          |                  |
| Concentration Risk Number of Customers                                                          | 1                     |
| Number Of Employees Hourly Workers [Member] | Unionized Employees Concentration Risk [Member]   |                  |
| **Risks and Uncertainties [Abstract]**                                                          |                  |
| Concentration Risk Number Of Employees                                                          | 4,700                 |
| concentration risk number of employees percentage                                               | 48.00%                |
| Number of Employees Salaried Workers [Member] | Unionized Employees Concentration Risk [Member] |                  |
| **Risks and Uncertainties [Abstract]**                                                          |                  |
| Concentration Risk Number Of Employees                                                          | 400                   |
| concentration risk number of employees percentage                                               | 4.00%                 |

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