Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Oct. 02, 2020	Jul. 03, 2020	Apr. 03, 2020	Jan. 03, 2020	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Oct. 02, 2020	Sep. 27, 2019	Sep. 28, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 850.5	$ 694.3	$ 794.5	$ 828.9	$ 878.9	$ 825.8	$ 779.4	$ 741.0	$ 3,168.2	$ 3,225.1	$ 2,919.1
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									3,118.9	3,205.7	2,919.1
Other
Disaggregation of Revenue [Line Items]
Revenues									49.3	19.4	0.0
Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									2,997.8	3,061.8	2,770.2
Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									121.1	143.9	148.9
Hardware
Disaggregation of Revenue [Line Items]
Revenues									1,341.2	1,532.3	1,366.6
Hardware | Other
Disaggregation of Revenue [Line Items]
Revenues									49.3	19.4	0.0
Hardware | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,211.0	1,393.6	1,231.5
Hardware | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									80.9	119.3	135.1
Software
Disaggregation of Revenue [Line Items]
Revenues									598.7	577.0	499.2
Software | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									596.4	574.0	495.4
Software | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									2.3	3.0	3.8
Service
Disaggregation of Revenue [Line Items]
Revenues									1,228.3	1,115.8	1,053.3
Service | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,190.4	1,094.2	1,043.3
Service | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									37.9	21.6	10.0
Americas
Disaggregation of Revenue [Line Items]
Revenues									1,534.7	1,527.4	1,436.9
Americas | Other
Disaggregation of Revenue [Line Items]
Revenues									24.4	6.1	0.0
Americas | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,449.7	1,451.3	1,351.3
Americas | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									60.6	70.0	85.6
EMEA
Disaggregation of Revenue [Line Items]
Revenues									1,000.1	1,073.4	942.8
EMEA | Other
Disaggregation of Revenue [Line Items]
Revenues									5.2	6.2	0.0
EMEA | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									942.1	1,000.9	883.2
EMEA | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									52.8	66.3	59.6
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenues									633.4	624.3	539.4
Asia Pacific | Other
Disaggregation of Revenue [Line Items]
Revenues									19.7	7.1	0.0
Asia Pacific | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									606.0	609.6	535.7
Asia Pacific | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									7.7	7.6	3.7
North America
Disaggregation of Revenue [Line Items]
Revenues									1,448.0	1,425.1	1,347.2
North America | Other
Disaggregation of Revenue [Line Items]
Revenues									24.4	5.9	0.0
North America | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,362.9	1,349.2	1,261.6
North America | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									60.7	70.0	85.6
International
Disaggregation of Revenue [Line Items]
Revenues									1,720.2	1,800.0	1,571.9
International | Other
Disaggregation of Revenue [Line Items]
Revenues									24.9	13.5	0.0
International | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,634.9	1,712.6	1,508.6
International | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									60.4	73.9	63.3
Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenues									1,506.9	1,664.7	1,434.8
Transferred at Point in Time | Other
Disaggregation of Revenue [Line Items]
Revenues									49.3	19.4	0.0
Transferred at Point in Time | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,455.3	1,642.3	1,431.0
Transferred at Point in Time | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									2.3	3.0	3.8
Transferred over Time
Disaggregation of Revenue [Line Items]
Revenues									1,661.3	1,560.4	1,484.3
Transferred over Time | Oncology Systems | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,542.5	1,419.5	1,339.2
Transferred over Time | Proton Solutions | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									$ 118.8	$ 140.9	$ 145.1