Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2012	Jul. 31, 2011	Apr. 30, 2012
Income taxes [Abstract]
Look back period in which no cumulative losses were reported, minimum			3 years
Look back period in which no cumulative losses were reported, maximum			6 years
Effective tax rate (in hundredths)	21.70%	40.60%	(106.70%)	41.80%
Incremental valuation allowance	$ 0.2		$ 15.4
Valuation allowance on deferred tax assets	16.0		16.0		15.2
Increase in valuation allowance	$ 0.2		$ 15.4