Business Segment Information - Sales by Geographical Region (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue
Sales	$ 475,475	$ 350,439	$ 321,482	$ 328,839	$ 471,198	$ 382,679	$ 337,339	$ 305,259	$ 1,476,235	$ 1,496,475	$ 1,202,898
United States
Disaggregation of Revenue
Sales									931,400	956,600	627,800
United Kingdom
Disaggregation of Revenue
Sales									207,900	218,200	240,700
Australia
Disaggregation of Revenue
Sales									164,300	163,500	166,700
Far East/Middle East
Disaggregation of Revenue
Sales									101,000	74,000	86,400
Other foreign countries
Disaggregation of Revenue
Sales									71,600	84,200	81,300
Cubic Transportation Systems
Disaggregation of Revenue
Sales									840,900	849,800	670,700
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales									495,600	488,300	273,400
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales									185,800	196,500	217,200
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales									131,000	131,900	130,700
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales									1,600	9,400	9,800
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales									26,900	23,700	39,600
Cubic Mission Solutions
Disaggregation of Revenue
Sales									337,100	328,800	207,000
Cubic Mission Solutions | United States
Disaggregation of Revenue
Sales									329,400	322,600	199,800
Cubic Mission Solutions | United Kingdom
Disaggregation of Revenue
Sales									3,200	1,600	2,700
Cubic Mission Solutions | Australia
Disaggregation of Revenue
Sales									1,800	2,600	2,400
Cubic Mission Solutions | Far East/Middle East
Disaggregation of Revenue
Sales									900	900	1,200
Cubic Mission Solutions | Other foreign countries
Disaggregation of Revenue
Sales									1,800	1,100	900
Cubic Global Defense Systems
Disaggregation of Revenue
Sales									298,200	317,900	325,200
Cubic Global Defense Systems | United States
Disaggregation of Revenue
Sales									106,400	145,700	154,600
Cubic Global Defense Systems | United Kingdom
Disaggregation of Revenue
Sales									18,900	20,100	20,800
Cubic Global Defense Systems | Australia
Disaggregation of Revenue
Sales									31,500	29,000	33,600
Cubic Global Defense Systems | Far East/Middle East
Disaggregation of Revenue
Sales									98,500	63,700	75,400
Cubic Global Defense Systems | Other foreign countries
Disaggregation of Revenue
Sales									$ 42,900	$ 59,400	$ 40,800