Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 04, 2020	Jun. 29, 2019	Jul. 04, 2020	Jun. 29, 2019	Apr. 04, 2020	Sep. 28, 2019	Mar. 30, 2019	Sep. 30, 2018	Sep. 29, 2018
Disaggregation of Revenue [Line Items]
Adoption of new revenue recognition standard (ASC 606) adjustment	$ (61,928)	$ (61,186)	$ (61,928)	$ (61,186)	$ (65,063)	$ (67,811)	$ (77,590)		$ (28,336)
Net sales	189,181	308,774	597,810	675,342
Diesel buses
Disaggregation of Revenue [Line Items]
Net sales	88,299	135,246	278,698	335,553
Alternative fuel buses
Disaggregation of Revenue [Line Items]
Net sales	80,975	141,939	245,766	260,340
Other
Disaggregation of Revenue [Line Items]
Net sales	11,583	15,486	30,931	32,061
Parts
Disaggregation of Revenue [Line Items]
Net sales	$ 8,324	$ 16,103	$ 42,415	$ 47,388
Accounting Standards Update 2014-09 | Cumulative Effect, Period of Adoption, Adjustment
Disaggregation of Revenue [Line Items]
Adoption of new revenue recognition standard (ASC 606) adjustment								$ (714)