Segment Information - Sales by Geographical Region (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue
Sales	$ 318,794	$ 328,839
United States
Disaggregation of Revenue
Sales	192,200	175,700
United Kingdom
Disaggregation of Revenue
Sales	55,900	52,100
Australia
Disaggregation of Revenue
Sales	38,700	44,200
Far East/Middle East
Disaggregation of Revenue
Sales	16,800	38,000
Other foreign countries
Disaggregation of Revenue
Sales	15,200	18,800
Cubic Transportation Systems
Disaggregation of Revenue
Sales	197,100	188,600
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales	111,900	96,700
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales	47,900	47,600
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales	29,100	36,400
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales	1,000	1,300
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales	7,200	6,600
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	121,700	140,200
Cubic Mission and Performance Solutions | United States
Disaggregation of Revenue
Sales	80,300	79,000
Cubic Mission and Performance Solutions | United Kingdom
Disaggregation of Revenue
Sales	8,000	4,500
Cubic Mission and Performance Solutions | Australia
Disaggregation of Revenue
Sales	9,600	7,800
Cubic Mission and Performance Solutions | Far East/Middle East
Disaggregation of Revenue
Sales	15,800	36,700
Cubic Mission and Performance Solutions | Other foreign countries
Disaggregation of Revenue
Sales	$ 8,000	$ 12,200