Income taxes Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Apr. 30, 2014 Rate	Oct. 31, 2012 Rate	Jan. 31, 2014
Income tax [Abstract]
effective income tax rate from continuing operations	23.20%	5.80%
Income tax examination, description	IRS began an audit of the fiscal year ended January 31, 2012
Open tax year	2010
Open tax years up to the amount of research tax credit generated	January 31, 2002 through January 31, 2009
Operating loss carryforwards			$ 9.3
Operating loss carryforwards, expiration dates	Jan. 31, 2030