Segment Information - Sales by Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue
Sales	$ 318,794	$ 328,839
Fixed Price
Disaggregation of Revenue
Sales	296,200	309,300
Other
Disaggregation of Revenue
Sales	22,600	19,500
Cubic Transportation Systems
Disaggregation of Revenue
Sales	197,100	188,600
Cubic Transportation Systems | Fixed Price
Disaggregation of Revenue
Sales	194,400	184,500
Cubic Transportation Systems | Other
Disaggregation of Revenue
Sales	2,700	4,100
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	121,700	140,200
Cubic Mission and Performance Solutions | Fixed Price
Disaggregation of Revenue
Sales	101,800	124,800
Cubic Mission and Performance Solutions | Other
Disaggregation of Revenue
Sales	$ 19,900	$ 15,400