Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 04, 2020	Mar. 30, 2019	Apr. 04, 2020	Mar. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 255,412	$ 211,642	$ 408,629	$ 366,568
Diesel buses
Disaggregation of Revenue [Line Items]
Net sales	113,649	110,487	190,399	200,307
Alternative fuel buses
Disaggregation of Revenue [Line Items]
Net sales	113,057	75,320	164,791	118,401
Other
Disaggregation of Revenue [Line Items]
Net sales	12,505	9,773	19,348	16,575
Parts
Disaggregation of Revenue [Line Items]
Net sales	$ 16,201	$ 16,062	$ 34,091	$ 31,285
Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Adoption of new revenue recognition standard (ASC 606) adjustment					$ (714)