Shareholders' equity - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Aug. 02, 2014
Equity [Abstract]
Prior period reclassification adjustment	$ 234.8

The amount of a reclassification adjustment made to prior period financial statement amounts.

No definition available.

Name:	us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration