Segment Information - Sales by Geographical Region (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue
Sales	$ 328,839	$ 305,259
United States
Disaggregation of Revenue
Sales	175,700	172,500
United Kingdom
Disaggregation of Revenue
Sales	52,100	55,600
Australia
Disaggregation of Revenue
Sales	44,200	35,500
Far East/Middle East
Disaggregation of Revenue
Sales	38,000	23,200
Other foreign countries
Disaggregation of Revenue
Sales	18,800	18,500
Cubic Transportation Systems
Disaggregation of Revenue
Sales	188,600	181,800
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales	96,700	91,300
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales	47,600	50,900
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales	36,400	29,800
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales	1,300	4,400
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales	6,600	5,400
Cubic Mission Solutions
Disaggregation of Revenue
Sales	58,700	46,400
Cubic Mission Solutions | United States
Disaggregation of Revenue
Sales	56,200	45,500
Cubic Mission Solutions | United Kingdom
Disaggregation of Revenue
Sales	200	400
Cubic Mission Solutions | Australia
Disaggregation of Revenue
Sales	1,300	100
Cubic Mission Solutions | Far East/Middle East
Disaggregation of Revenue
Sales	400	400
Cubic Mission Solutions | Other foreign countries
Disaggregation of Revenue
Sales	600
Cubic Global Defense Systems
Disaggregation of Revenue
Sales	81,500	77,100
Cubic Global Defense Systems | United States
Disaggregation of Revenue
Sales	22,800	35,700
Cubic Global Defense Systems | United Kingdom
Disaggregation of Revenue
Sales	4,300	4,300
Cubic Global Defense Systems | Australia
Disaggregation of Revenue
Sales	6,500	5,600
Cubic Global Defense Systems | Far East/Middle East
Disaggregation of Revenue
Sales	36,300	18,400
Cubic Global Defense Systems | Other foreign countries
Disaggregation of Revenue
Sales	$ 11,600	$ 13,100