Segment Information - Sales by Contract Type (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
Fixed Price
Disaggregation of Revenue
Sales	321,800	302,100	618,000	611,400
Other
Disaggregation of Revenue
Sales	21,600	19,400	44,200	38,900
Cubic Transportation Systems
Disaggregation of Revenue
Sales	217,400	197,600	414,500	386,200
Cubic Transportation Systems | Fixed Price
Disaggregation of Revenue
Sales	214,600	194,800	409,000	379,300
Cubic Transportation Systems | Other
Disaggregation of Revenue
Sales	2,800	2,800	5,500	6,900
Cubic Mission and Performance Solutions
Disaggregation of Revenue
Sales	126,000	123,900	247,700	264,100
Cubic Mission and Performance Solutions | Fixed Price
Disaggregation of Revenue
Sales	107,200	107,300	209,000	232,100
Cubic Mission and Performance Solutions | Other
Disaggregation of Revenue
Sales	$ 18,800	$ 16,600	$ 38,700	$ 32,000