Note 11 - Industry Segmentation and Sales by Major Customer - Schedule of Revenue by Major Customers by Reporting Segments (Details) (Parentheticals)	Apr. 30, 2018	Apr. 30, 2017
Aerospace Products [Member] \| Customer Concentration Risk [Member] \| Sales Revenue, Net [Member]
Number of customers	1	1

Represents information about number of major customers.

No definition available.

Name:	buks_NumberOfMajorCustomers
Namespace Prefix:	buks_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant