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Segment Information - Operating Results Information for Each Reportable Segment (Detail) - 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
Segment Reporting Information [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
Operating earnings									386.2	437.4	297.8
Interest income, net									6.3	10.5	2.9
Other income									28.3	4.2	2.4
Earnings from continuing operations before taxes									420.8	452.1	303.1
Depreciation & Amortization									93.0	72.7	76.9
Total Assets	4,101.7				3,252.7				4,101.7	3,252.7
Discontinued Operations
Segment Reporting Information [Line Items]
Depreciation & Amortization									0.0	0.0	6.6
Total Assets	0.0				2.3				0.0	2.3
Corporate Segment
Segment Reporting Information [Line Items]
Operating earnings									(46.6)	(64.5)	(85.5)
Depreciation & Amortization									23.8	22.7	24.8
Total Assets	1,178.9				1,124.2				1,178.9	1,124.2
Operating Segments
Segment Reporting Information [Line Items]
Revenues									3,205.7	2,919.1	2,619.3
Operating earnings									458.6	501.9	383.3
Operating Segments \| Oncology Systems
Segment Reporting Information [Line Items]
Revenues									3,061.8	2,770.2	2,436.8
Operating earnings									555.9	553.4	479.0
Depreciation & Amortization									49.6	42.1	39.4
Total Assets	2,339.1				1,746.4				2,339.1	1,746.4
Operating Segments \| Proton Solutions
Segment Reporting Information [Line Items]
Revenues									143.9	148.9	182.5
Operating earnings									(97.3)	(51.5)	(95.7)
Depreciation & Amortization									4.8	7.9	6.1
Total Assets	275.9				379.8				275.9	379.8
Operating Segments \| Reportable Segments
Segment Reporting Information [Line Items]
Depreciation & Amortization									54.4	50.0	45.5
Total Assets	2,615.0				2,126.2				2,615.0	2,126.2
Operating Segments \| Other
Segment Reporting Information [Line Items]
Revenues									19.4	0.0	0.0
Operating earnings									(25.8)	0.0	0.0
Depreciation & Amortization									14.8	0.0	$ 0.0
Total Assets	$ 307.8				$ 0.0				$ 307.8	$ 0.0