Investments - Contractual Maturities of Debt Securities Classified as Trading (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Apr. 30, 2019
Debt and Equity Securities, FV-NI [Line Items]
Total debt securities	$ 3,104	$ 16,192
Tax-exempt state and municipal bonds
Debt and Equity Securities, FV-NI [Line Items]
Due within one year	2,403	13,708
Due within two years	701	2,484
Due within three years	0	0
Due after three years	0	0
Total debt securities	$ 3,104	$ 16,192

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant