Investments - Available-for Sale Debt Securities by Contractual Maturity (Details) - Debt Securities, Available-For-Sale, Excluding Money Market Accounts - USD ($) $ in Thousands	Jan. 31, 2021	Apr. 30, 2020
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract]
AFS Debt Maturities, Within one year, Amortized Cost	$ 163,865	$ 212,449
AFS Debt Maturities, After one year through five years, Amortized Cost	0	0
Amortized Cost	163,865	212,449
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
AFS Debt Maturities, Within one year, Fair Value	163,880	212,873
AFS Debt Maturities, After one year through five years, Fair Value	0	0
AFS Debt Maturities, Total Fair Value	$ 163,880	$ 212,873

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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