Segment Information - Geographic Information (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
Property, plant and equipment, net	344.9				311.5				344.9	311.5
Depreciation & Amortization									98.9	93.0	72.7
Total Assets	4,462.2				4,101.7				4,462.2	4,101.7
Transferred at Point in Time
Segment Reporting Information [Line Items]
Revenues									1,506.9	1,664.7	1,434.8
Transferred over Time
Segment Reporting Information [Line Items]
Revenues									1,661.3	1,560.4	1,484.3
Operating Segments
Segment Reporting Information [Line Items]
Revenues									3,118.9	3,205.7	2,919.1
Depreciation & Amortization									57.2	54.4	50.0
Total Assets	2,601.6				2,615.0				2,601.6	2,615.0
Operating Segments | Oncology Systems
Segment Reporting Information [Line Items]
Revenues									2,997.8	3,061.8	2,770.2
Depreciation & Amortization									53.2	49.6	42.1
Total Assets	2,315.7				2,339.1				2,315.7	2,339.1
Operating Segments | Oncology Systems | Transferred at Point in Time
Segment Reporting Information [Line Items]
Revenues									1,455.3	1,642.3	1,431.0
Operating Segments | Oncology Systems | Transferred over Time
Segment Reporting Information [Line Items]
Revenues									1,542.5	1,419.5	1,339.2
Operating Segments | Proton Solutions
Segment Reporting Information [Line Items]
Revenues									121.1	143.9	148.9
Depreciation & Amortization									4.0	4.8	7.9
Total Assets	285.9				275.9				285.9	275.9
Operating Segments | Proton Solutions | Transferred at Point in Time
Segment Reporting Information [Line Items]
Revenues									2.3	3.0	3.8
Operating Segments | Proton Solutions | Transferred over Time
Segment Reporting Information [Line Items]
Revenues									118.8	140.9	145.1
Other
Segment Reporting Information [Line Items]
Revenues									49.3	19.4	0.0
Depreciation & Amortization									11.5	14.8	0.0
Total Assets	314.6				307.8				314.6	307.8
Other | Transferred at Point in Time
Segment Reporting Information [Line Items]
Revenues									49.3	19.4	0.0
Corporate
Segment Reporting Information [Line Items]
Depreciation & Amortization									30.2	23.8	22.7
Total Assets	1,546.0				1,178.9				1,546.0	1,178.9
United States
Segment Reporting Information [Line Items]
Revenues									1,393.5	1,384.3	1,308.3
Property, plant and equipment, net	163.6				155.3				163.6	155.3
Other countries
Segment Reporting Information [Line Items]
Revenues									1,774.7	1,840.8	$ 1,610.8
Property, plant and equipment, net	$ 181.3				$ 156.2				$ 181.3	$ 156.2