Segment Information - Revenue Recognition Method (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
Point in time
Disaggregation of Revenue
Sales	41,700	43,100	85,500	100,200
Over time
Disaggregation of Revenue
Sales	301,700	278,400	576,700	550,100
Cubic Transportation Systems
Disaggregation of Revenue
Sales	217,400	197,600	414,500	386,200
Cubic Transportation Systems | Point in time
Disaggregation of Revenue
Sales	20,800	28,100	39,600	45,800
Cubic Transportation Systems | Over time
Disaggregation of Revenue
Sales	196,600	169,500	374,900	340,400
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	126,000	123,900	247,700	264,100
Cubic Mission and Performance Solutions | Point in time
Disaggregation of Revenue
Sales	20,900	15,000	45,900	54,400
Cubic Mission and Performance Solutions | Over time
Disaggregation of Revenue
Sales	$ 105,100	$ 108,900	$ 201,800	$ 209,700