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Segment Information - Operating Results Information for Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 28, 2018	Sep. 29, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 801.6	$ 709.1	$ 729.9	$ 678.5	$ 721.8	$ 632.8	$ 663.2	$ 601.5	$ 2,919.1	$ 2,619.3	$ 2,593.7
Operating earnings									441.6	300.2	426.8
Interest income, net									10.5	2.9	5.6
Earnings from continuing operations before taxes									452.1	303.1	432.4
Depreciation & Amortization									72.7	76.9	79.8
Total Assets	3,252.7				3,294.4				3,252.7	3,294.4
Discontinued Operations
Segment Reporting Information [Line Items]
Depreciation & Amortization									0.0	6.6	16.0
Total Assets	2.3				11.1				2.3	11.1
Oncology Systems
Segment Reporting Information [Line Items]
Revenues									2,770.2	2,436.8	2,430.6
Proton Solutions
Segment Reporting Information [Line Items]
Revenues									148.9	182.5	162.6
Corporate Segment
Segment Reporting Information [Line Items]
Operating earnings									(60.3)	(83.1)	(33.4)
Depreciation & Amortization									22.7	24.8	28.3
Total Assets	1,124.2				1,360.2				1,124.2	1,360.2
Operating Segments
Segment Reporting Information [Line Items]
Revenues									2,919.1	2,619.3	2,593.2
Operating earnings									501.9	383.3	465.8
Operating Segments \| Oncology Systems
Segment Reporting Information [Line Items]
Revenues									2,770.2	2,436.8	2,430.6
Operating earnings									553.4	479.0	510.4
Depreciation & Amortization									42.1	39.4	28.5
Total Assets	1,746.4				1,568.7				1,746.4	1,568.7
Operating Segments \| Proton Solutions
Segment Reporting Information [Line Items]
Revenues									148.9	182.5	162.6
Operating earnings									(51.5)	(95.7)	(44.6)
Depreciation & Amortization									7.9	6.1	6.8
Total Assets	379.8				354.4				379.8	354.4
Operating Segments \| Reportable Segments
Segment Reporting Information [Line Items]
Depreciation & Amortization									50.0	45.5	35.3
Total Assets	2,126.2				1,923.1				2,126.2	1,923.1
Operating Segments \| Other
Segment Reporting Information [Line Items]
Revenues									0.0	0.0	0.5
Operating earnings									0.0	0.0	(5.6)
Depreciation & Amortization									0.0	0.0	$ 0.2
Total Assets	$ 0.0				$ 0.0				$ 0.0	$ 0.0