Summary of significant accounting policies (Details)	Jan. 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
Concentration Risk, Percentage	10.00%
Number Of Employees Hourly Workers [Member] | Unionized Employees Concentration Risk [Member]
Risks and Uncertainties [Abstract]
Concentration Risk Number Of Employees	5,600
Concentration Risk, Percentage	53.00%
Number of Employees Salaried Workers [Member] | Unionized Employees Concentration Risk [Member]
Risks and Uncertainties [Abstract]
Concentration Risk Number Of Employees	400
Concentration Risk, Percentage	5.00%