Segment Information - Sales by End Customer (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue
Sales	$ 343,408	$ 321,482	$ 662,202	$ 650,321
U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	197,900	165,200	384,800	340,700
Other
Disaggregation of Revenue
Sales	145,500	156,300	277,400	309,600
Cubic Transportation Systems
Disaggregation of Revenue
Sales	217,400	197,600	414,500	386,200
Cubic Transportation Systems | U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	115,400	104,000	223,000	197,500
Cubic Transportation Systems | Other
Disaggregation of Revenue
Sales	102,000	93,600	191,500	188,700
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	126,000	123,900	247,700	264,100
Cubic Mission and Performance Solutions | U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	82,500	61,200	161,800	143,200
Cubic Mission and Performance Solutions | Other
Disaggregation of Revenue
Sales	$ 43,500	$ 62,700	$ 85,900	$ 120,900