Segment Information - Operating Results Information for Each Reportable Segment (Detail) - 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
Segment Reporting Information [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 earnings									323.2	386.2	437.4
Interest income, net									(3.6)	6.3	10.5
Other income, net									38.7	28.3	4.2
Earnings before taxes									358.3	420.8	452.1
Operating Segments
Segment Reporting Information [Line Items]
Revenues									3,118.9	3,205.7	2,919.1
Operating earnings									452.5	458.6	501.9
Operating Segments | Oncology Systems
Segment Reporting Information [Line Items]
Revenues									2,997.8	3,061.8	2,770.2
Operating earnings									518.0	555.9	553.4
Operating Segments | Proton Solutions
Segment Reporting Information [Line Items]
Revenues									121.1	143.9	148.9
Operating earnings									(65.5)	(97.3)	(51.5)
Other
Segment Reporting Information [Line Items]
Revenues									49.3	19.4	0.0
Operating earnings									(9.6)	(25.8)	0.0
Corporate
Segment Reporting Information [Line Items]
Operating earnings									$ (119.7)	$ (46.6)	$ (64.5)