Organization and Significant Accounting Policies - Revenue by Category (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2020	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Disaggregation of revenue
Revenue	$ 135,223	$ 61,891	$ 83,271	$ 86,911	$ 87,930	$ 75,322	$ 72,979	$ 78,043	$ 367,296	$ 314,274	$ 268,424
Contract Liability
Disaggregation of revenue
Revenue									1,670	1,587	977
Domestic
Disaggregation of revenue
Revenue									201,046	151,124	142,158
International
Disaggregation of revenue
Revenue									166,250	163,150	126,266
U.S. government
Disaggregation of revenue
Revenue									225,341	182,586	156,996
Non-U.S. government
Disaggregation of revenue
Revenue									141,955	131,688	111,428
FFP
Disaggregation of revenue
Revenue									269,917	224,090	212,976
CPFF
Disaggregation of revenue
Revenue									94,176	89,485	55,203
T&M
Disaggregation of revenue
Revenue									3,203	699	245
Small UAS
Disaggregation of revenue
Revenue									225,888	183,157	167,534
TMS
Disaggregation of revenue
Revenue									63,781	65,087	63,406
HAPS
Disaggregation of revenue
Revenue									60,864	55,407	29,593
Other
Disaggregation of revenue
Revenue									$ 16,763	$ 10,623	$ 7,891