Segment Information - Sales by Geographical Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue
Sales	$ 350,439	$ 382,679	$ 1,000,760	$ 1,025,277
United States
Disaggregation of Revenue
Sales	233,900	254,500	582,100	628,700
United Kingdom
Disaggregation of Revenue
Sales	48,600	55,400	155,700	169,200
Australia
Disaggregation of Revenue
Sales	39,000	39,300	120,500	112,300
Far East/Middle East
Disaggregation of Revenue
Sales	13,000	16,000	87,000	55,000
Other foreign countries
Disaggregation of Revenue
Sales	15,900	17,500	55,500	60,100
Cubic Transportation Systems
Disaggregation of Revenue
Sales	215,500	212,700	601,800	595,200
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales	134,700	128,900	342,500	327,300
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales	43,300	47,500	140,800	151,600
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales	30,200	30,900	96,700	91,100
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales	300	700	2,200	8,500
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales	7,000	4,700	19,600	16,700
Cubic Mission Solutions
Disaggregation of Revenue
Sales	69,000	95,000	167,200	203,300
Cubic Mission Solutions | United States
Disaggregation of Revenue
Sales	67,000	94,000	161,300	200,100
Cubic Mission Solutions | United Kingdom
Disaggregation of Revenue
Sales	600	200	1,300	1,300
Cubic Mission Solutions | Australia
Disaggregation of Revenue
Sales	700	300	2,400	500
Cubic Mission Solutions | Far East/Middle East
Disaggregation of Revenue
Sales	300	100	800	700
Cubic Mission Solutions | Other foreign countries
Disaggregation of Revenue
Sales	400	400	1,400	700
Cubic Global Defense Systems
Disaggregation of Revenue
Sales	65,900	75,000	231,800	226,800
Cubic Global Defense Systems | United States
Disaggregation of Revenue
Sales	32,200	31,600	78,300	101,300
Cubic Global Defense Systems | United Kingdom
Disaggregation of Revenue
Sales	4,700	7,700	13,600	16,300
Cubic Global Defense Systems | Australia
Disaggregation of Revenue
Sales	8,100	8,100	21,400	20,700
Cubic Global Defense Systems | Far East/Middle East
Disaggregation of Revenue
Sales	12,400	15,200	84,000	45,800
Cubic Global Defense Systems | Other foreign countries
Disaggregation of Revenue
Sales	$ 8,500	$ 12,400	$ 34,500	$ 42,700