Segment Information - Sales by End Customer (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue
Sales	$ 318,794	$ 328,839
U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	186,900	175,500
Other
Disaggregation of Revenue
Sales	131,900	153,300
Cubic Transportation Systems
Disaggregation of Revenue
Sales	197,100	188,600
Cubic Transportation Systems | U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	107,600	93,500
Cubic Transportation Systems | Other
Disaggregation of Revenue
Sales	89,500	95,100
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	121,700	140,200
Cubic Mission and Performance Solutions | U.S. Federal Government and State and Local Municipalities
Disaggregation of Revenue
Sales	79,300	82,000
Cubic Mission and Performance Solutions | Other
Disaggregation of Revenue
Sales	$ 42,400	$ 58,200