Summary of significant accounting policies (Details)	Jul. 31, 2012 segments
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%