SEGMENT INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Jan. 03, 2020	Dec. 28, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 828.9	$ 741.0
Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	400.9	366.6
Products and Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	428.0	374.4
Americas
Disaggregation of Revenue [Line Items]
Revenues	402.2	350.4
EMEA
Disaggregation of Revenue [Line Items]
Revenues	272.9	251.0
APAC
Disaggregation of Revenue [Line Items]
Revenues	153.8	139.6
North America
Disaggregation of Revenue [Line Items]
Revenues	378.5	328.7
International
Disaggregation of Revenue [Line Items]
Revenues	450.4	412.3
Hardware
Disaggregation of Revenue [Line Items]
Revenues	359.0	347.2
Software
Disaggregation of Revenue [Line Items]
Revenues	148.8	131.2
Service
Disaggregation of Revenue [Line Items]
Revenues	321.1	262.6
Oncology Systems | Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	382.0	366.6
Oncology Systems | Products and Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	400.4	335.9
Oncology Systems | Americas
Disaggregation of Revenue [Line Items]
Revenues	378.9	330.8
Oncology Systems | EMEA
Disaggregation of Revenue [Line Items]
Revenues	259.8	234.5
Oncology Systems | APAC
Disaggregation of Revenue [Line Items]
Revenues	143.7	137.2
Oncology Systems | North America
Disaggregation of Revenue [Line Items]
Revenues	355.2	309.1
Oncology Systems | International
Disaggregation of Revenue [Line Items]
Revenues	427.2	393.4
Oncology Systems | Hardware
Disaggregation of Revenue [Line Items]
Revenues	320.0	313.6
Oncology Systems | Software
Disaggregation of Revenue [Line Items]
Revenues	148.8	131.2
Oncology Systems | Service
Disaggregation of Revenue [Line Items]
Revenues	313.6	257.7
Proton Solutions | Products and Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	27.6	38.5
Proton Solutions | Americas
Disaggregation of Revenue [Line Items]
Revenues	16.9	19.6
Proton Solutions | EMEA
Disaggregation of Revenue [Line Items]
Revenues	10.3	16.5
Proton Solutions | APAC
Disaggregation of Revenue [Line Items]
Revenues	0.4	2.4
Proton Solutions | North America
Disaggregation of Revenue [Line Items]
Revenues	16.9	19.6
Proton Solutions | International
Disaggregation of Revenue [Line Items]
Revenues	10.7	18.9
Proton Solutions | Hardware
Disaggregation of Revenue [Line Items]
Revenues	20.1	33.6
Proton Solutions | Software
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Proton Solutions | Service
Disaggregation of Revenue [Line Items]
Revenues	7.5	4.9
Other | Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	18.9	0.0
Other | Americas
Disaggregation of Revenue [Line Items]
Revenues	6.4	0.0
Other | EMEA
Disaggregation of Revenue [Line Items]
Revenues	2.8	0.0
Other | APAC
Disaggregation of Revenue [Line Items]
Revenues	9.7	0.0
Other | North America
Disaggregation of Revenue [Line Items]
Revenues	6.4	0.0
Other | International
Disaggregation of Revenue [Line Items]
Revenues	12.5	0.0
Other | Hardware
Disaggregation of Revenue [Line Items]
Revenues	$ 18.9	$ 0.0