Note 12 - Industry Segmentation and Sales by Major Customer - Schedule of Revenue by Major Customers by Reporting Segments (Details) (Parentheticals)	Apr. 30, 2020	Apr. 30, 2019
Aerospace Products [Member] \| Customer Concentration Risk [Member] \| Revenue Benchmark [Member]
Number of customers	2	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