Goodwill and Intangible Assets - Activity of Goodwill by Reportable Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Jun. 28, 2019	Dec. 28, 2018	Jun. 29, 2018	Sep. 27, 2019	Sep. 28, 2018	Sep. 29, 2017
Goodwill [Roll Forward]
Balance, beginning		$ 293,600		$ 293,600	$ 222,600
Business combinations				372,300	72,100
Impairment charges	$ 50,500			50,500	0	$ 0
Measurement period adjustment to a business combination in prior year		(4,800)		(2,200)
Foreign currency translation adjustments				(1,000)	(1,100)
Balance, ending				612,200	293,600	222,600
Oncology Systems
Goodwill [Roll Forward]
Balance, beginning		242,100		242,100	170,200
Business combinations				208,000	72,100
Measurement period adjustment to a business combination in prior year				(2,200)
Foreign currency translation adjustments					(200)
Balance, ending				447,900	242,100	170,200
Proton Solutions
Goodwill [Roll Forward]
Balance, beginning		51,500		51,500	52,400
Impairment charges			$ 50,500	50,500
Foreign currency translation adjustments				(1,000)	(900)
Balance, ending				0	51,500	$ 52,400
Other
Goodwill [Roll Forward]
Balance, beginning		$ 0		0
Business combinations				164,300
Balance, ending				$ 164,300	$ 0