Segment Information - Sales by Geographical Region (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
United States
Disaggregation of Revenue
Sales	207,400	172,400	399,600	348,100
United Kingdom
Disaggregation of Revenue
Sales	61,900	55,000	117,800	107,100
Australia
Disaggregation of Revenue
Sales	41,800	37,300	80,500	81,500
Far East/Middle East
Disaggregation of Revenue
Sales	16,800	36,000	33,600	74,000
Other foreign countries
Disaggregation of Revenue
Sales	15,500	20,800	30,700	39,600
Cubic Transportation Systems
Disaggregation of Revenue
Sales	217,400	197,600	414,500	386,200
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales	123,600	111,000	235,500	207,700
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales	54,300	49,900	102,200	97,500
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales	32,300	30,100	61,400	66,500
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales	500	600	1,500	1,900
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales	6,700	6,000	13,900	12,600
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	126,000	123,900	247,700	264,100
Cubic Mission and Performance Solutions | United States
Disaggregation of Revenue
Sales	83,800	61,400	164,100	140,400
Cubic Mission and Performance Solutions | United Kingdom
Disaggregation of Revenue
Sales	7,600	5,100	15,600	9,600
Cubic Mission and Performance Solutions | Australia
Disaggregation of Revenue
Sales	9,500	7,200	19,100	15,000
Cubic Mission and Performance Solutions | Far East/Middle East
Disaggregation of Revenue
Sales	16,300	35,400	32,100	72,100
Cubic Mission and Performance Solutions | Other foreign countries
Disaggregation of Revenue
Sales	$ 8,800	$ 14,800	$ 16,800	$ 27,000