Summary of significant accounting policies (Details)	Apr. 30, 2012 customers	Apr. 30, 2011
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		12.00%
Number Of Employees Hourly Workers [Member] | Unionized Employees Concentration Risk [Member]
Risks and Uncertainties [Abstract]
Concentration Risk Number Of Employees	5,400
Concentration Risk, Percentage	51.92308%
Number of Employees Salaried Workers [Member] | Unionized Employees Concentration Risk [Member]
Risks and Uncertainties [Abstract]
Concentration Risk Number Of Employees	400
Concentration Risk, Percentage	4.30108%