Organization and Significant Accounting Policies - Concentration of Credit Risk (Details)	12 Months Ended
Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Customer concentration \| Sales Revenue \| US Government Agencies
Concentration of Credit Risk
Concentration Risk, Percentage	61.00%	58.00%	58.00%
Customer concentration \| Sales Revenue \| US Army
Concentration of Credit Risk
Concentration Risk, Percentage	32.00%	28.00%	19.00%
Credit concentration \| Accounts receivable balances \| US Government Agencies
Concentration of Credit Risk
Concentration Risk, Percentage	62.00%	26.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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