Investments - Auction rate securities (Details) - Auction rate securities	12 Months Ended
Apr. 30, 2019 item
Investments
Number of available-for-sale securities	2
Minimum
Available For Sale Securities
Maturity period of available-for-sale securities	1 year
Maximum
Available For Sale Securities
Maturity period of available-for-sale securities	16 years

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant