Segment Information - Sales by Geographical Region (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue
Sales	$ 321,482	$ 337,339	$ 650,321	$ 642,598
United States
Disaggregation of Revenue
Sales	172,400	201,400	348,100	373,900
United Kingdom
Disaggregation of Revenue
Sales	55,000	58,200	107,100	113,800
Australia
Disaggregation of Revenue
Sales	37,300	37,600	81,500	73,100
Far East/Middle East
Disaggregation of Revenue
Sales	36,000	16,000	74,000	39,200
Other foreign countries
Disaggregation of Revenue
Sales	20,800	24,100	39,600	42,600
Cubic Transportation Systems
Disaggregation of Revenue
Sales	197,600	200,700	386,200	382,500
Cubic Transportation Systems | United States
Disaggregation of Revenue
Sales	111,000	106,900	207,700	198,200
Cubic Transportation Systems | United Kingdom
Disaggregation of Revenue
Sales	49,900	53,200	97,500	104,100
Cubic Transportation Systems | Australia
Disaggregation of Revenue
Sales	30,100	30,400	66,500	60,200
Cubic Transportation Systems | Far East/Middle East
Disaggregation of Revenue
Sales	600	3,500	1,900	7,900
Cubic Transportation Systems | Other foreign countries
Disaggregation of Revenue
Sales	6,000	6,700	12,600	12,100
Cubic Mission Solutions
Disaggregation of Revenue
Sales	39,500	61,900	98,200	108,300
Cubic Mission Solutions | United States
Disaggregation of Revenue
Sales	38,100	60,500	94,300	106,000
Cubic Mission Solutions | United Kingdom
Disaggregation of Revenue
Sales	500	700	700	1,100
Cubic Mission Solutions | Australia
Disaggregation of Revenue
Sales	400	100	1,700	200
Cubic Mission Solutions | Far East/Middle East
Disaggregation of Revenue
Sales	100	200	500	600
Cubic Mission Solutions | Other foreign countries
Disaggregation of Revenue
Sales	400	400	1,000	400
Cubic Global Defense Systems
Disaggregation of Revenue
Sales	84,400	74,700	165,900	151,800
Cubic Global Defense Systems | United States
Disaggregation of Revenue
Sales	23,300	34,000	46,100	69,700
Cubic Global Defense Systems | United Kingdom
Disaggregation of Revenue
Sales	4,600	4,300	8,900	8,600
Cubic Global Defense Systems | Australia
Disaggregation of Revenue
Sales	6,800	7,100	13,300	12,700
Cubic Global Defense Systems | Far East/Middle East
Disaggregation of Revenue
Sales	35,300	12,300	71,600	30,700
Cubic Global Defense Systems | Other foreign countries
Disaggregation of Revenue
Sales	$ 14,400	$ 17,000	$ 26,000	$ 30,100