Segment Information - Revenue Recognition Method (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue
Sales	$ 318,794	$ 328,839
Point in time
Disaggregation of Revenue
Sales	43,800	57,000
Over time
Disaggregation of Revenue
Sales	275,000	271,800
Cubic Transportation Systems
Disaggregation of Revenue
Sales	197,100	188,600
Cubic Transportation Systems | Point in time
Disaggregation of Revenue
Sales	18,800	17,700
Cubic Transportation Systems | Over time
Disaggregation of Revenue
Sales	178,300	170,900
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	121,700	140,200
Cubic Mission and Performance Solutions | Point in time
Disaggregation of Revenue
Sales	25,000	39,300
Cubic Mission and Performance Solutions | Over time
Disaggregation of Revenue
Sales	$ 96,700	$ 100,900