Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 27, 2019	Sep. 28, 2018	Sep. 29, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 878.9	$ 825.8	$ 779.4	$ 741.0	$ 801.6	$ 709.1	$ 729.9	$ 678.5	$ 3,225.1	$ 2,919.1	$ 2,619.3
Hardware
Disaggregation of Revenue [Line Items]
Revenues									1,532.3	1,366.6	1,211.9
Hardware | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,393.6	1,231.5	1,039.4
Hardware | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									119.3	135.1	172.5
Hardware | Other
Disaggregation of Revenue [Line Items]
Revenues									19.4	0.0	0.0
Software
Disaggregation of Revenue [Line Items]
Revenues									577.0	499.2	462.9
Software | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									574.0	495.4	462.9
Software | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									3.0	3.8	0.0
Service
Disaggregation of Revenue [Line Items]
Revenues									1,115.8	1,053.3	944.5
Service | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,094.2	1,043.3	934.5
Service | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									21.6	10.0	10.0
Americas
Disaggregation of Revenue [Line Items]
Revenues									1,527.4	1,436.9	1,344.6
Americas | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,451.3	1,351.3	1,256.8
Americas | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									70.0	85.6	87.8
Americas | Other
Disaggregation of Revenue [Line Items]
Revenues									6.1	0.0	0.0
EMEA
Disaggregation of Revenue [Line Items]
Revenues									1,073.4	942.8	759.2
EMEA | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,000.9	883.2	691.1
EMEA | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									66.3	59.6	68.1
EMEA | Other
Disaggregation of Revenue [Line Items]
Revenues									6.2	0.0	0.0
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenues									624.3	539.4	515.5
Asia Pacific | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									609.6	535.7	488.9
Asia Pacific | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									7.6	3.7	26.6
Asia Pacific | Other
Disaggregation of Revenue [Line Items]
Revenues									7.1	0.0	0.0
North America
Disaggregation of Revenue [Line Items]
Revenues									1,425.1	1,347.2	1,267.8
North America | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,349.2	1,261.6	1,180.0
North America | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									70.0	85.6	87.8
North America | Other
Disaggregation of Revenue [Line Items]
Revenues									5.9	0.0	0.0
International
Disaggregation of Revenue [Line Items]
Revenues									1,800.0	1,571.9	1,351.5
International | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,712.6	1,508.6	1,256.8
International | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									73.9	63.3	94.7
International | Other
Disaggregation of Revenue [Line Items]
Revenues									13.5	0.0	0.0
Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenues									1,664.7	1,434.8	1,221.5
Transferred at Point in Time | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,642.3	1,431.0	1,221.5
Transferred at Point in Time | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									3.0	3.8	0.0
Transferred at Point in Time | Other
Disaggregation of Revenue [Line Items]
Revenues									19.4	0.0	0.0
Transferred over Time
Disaggregation of Revenue [Line Items]
Revenues									1,560.4	1,484.3	1,397.8
Transferred over Time | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues									1,419.5	1,339.2	1,215.3
Transferred over Time | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues									140.9	145.1	182.5
Transferred over Time | Other
Disaggregation of Revenue [Line Items]
Revenues									$ 0.0	$ 0.0	$ 0.0