Income taxes (Detail)	3 Months Ended		6 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate	3.10%	3.60%	2.50%	3.70%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes			34.00%