Business Segment Information - Financial data (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
Segment Information
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
Operating income (loss)	73,286	$ 24,706	$ (29,934)	$ (6,483)	58,619	$ 34,725	$ (6,541)	$ (566)	61,575	86,237	24,382
Assets	2,324,221				1,847,170				2,324,221	1,847,170	1,304,900
Depreciation and amortization									88,482	64,742	46,600
Capital expenditures									49,247	49,084	31,696
Unallocated corporate expenses
Segment Information
Operating income (loss)									(55,600)	(21,800)	(52,500)
Assets	222,800				189,300				222,800	189,300	201,700
Depreciation and amortization									4,000	3,900	3,700
Capital expenditures									15,400	26,900	17,000
Cubic Transportation Systems
Segment Information
Sales									840,900	849,800	670,700
Cubic Transportation Systems | Operating segments
Segment Information
Sales									840,900	849,800	670,700
Operating income (loss)									121,000	77,200	60,400
Assets	1,050,300				825,800				1,050,300	825,800	390,200
Depreciation and amortization									29,500	30,700	12,000
Capital expenditures									6,300	6,600	3,200
Cubic Mission Solutions
Segment Information
Sales									337,100	328,800	207,000
Cubic Mission Solutions | Operating segments
Segment Information
Sales									337,100	328,800	207,000
Operating income (loss)									(26,700)	7,800	(100)
Assets	703,600				437,900				703,600	437,900	352,900
Depreciation and amortization									47,800	23,300	22,400
Capital expenditures									15,800	11,100	2,100
Cubic Global Defense Systems
Segment Information
Sales									298,200	317,900	325,200
Cubic Global Defense Systems | Operating segments
Segment Information
Sales									298,200	317,900	325,200
Operating income (loss)									22,900	23,000	16,600
Assets	$ 347,500				$ 394,200				347,500	394,200	360,100
Depreciation and amortization									7,200	6,800	8,500
Capital expenditures									$ 11,700	$ 4,500	$ 9,400