Condensed consolidating financial information - Narrative (Details)	Apr. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Parent's ownership in guarantor subsidiary (percent)	100.00%

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

No definition available.

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant