PROTON SOLUTIONS LOANS AND INVESTMENTS - Additional Information (Detail) € in Millions, $ in Millions			1 Months Ended					3 Months Ended		12 Months Ended	13 Months Ended
	Dec. 06, 2017 USD ($) tranche	Jul. 31, 2015 USD ($)	Sep. 28, 2018 USD ($)	Aug. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Jul. 31, 2017 EUR (€)	Dec. 28, 2018 USD ($) board_seat	Dec. 29, 2017 USD ($)	Sep. 29, 2017 USD ($)	Jul. 31, 2018 USD ($)	Jan. 01, 2023 USD ($)	Jan. 01, 2022 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)
Long term notes receivable and available for sale securities [Line Items]
Payments to acquire available-for-sale securities								$ 0.0	$ 6.0
Fair Value			$ 62.5					63.7
Loss from equity investments								$ (2.2)	$ 0.0
APTC Securities
Long term notes receivable and available for sale securities [Line Items]
Payments to acquire available-for-sale securities					$ 6.0
Interest rate (as a percent)								8.50%
C P T C
Long term notes receivable and available for sale securities [Line Items]
Equity ownership interest (percent)	47.08%							47.08%
Fair value of CPTC's equity	$ 20.1
Equity method investment	9.5		2.2					$ 0.0
Number of CPTC board seats held by the Company | board_seat								1
Number of CPTC board seats | board_seat								5
RPTC Securities
Long term notes receivable and available for sale securities [Line Items]
Claims filed against assets of Company in bankruptcy | €							€ 89.0
Accounts receivable, includes unbilled accounts receivable			4.5					$ 4.5
Georgia Proton Therapy Center
Long term notes receivable and available for sale securities [Line Items]
Accounts receivable, includes unbilled accounts receivable			12.5					8.6
Unbilled receivables from variable interest entity			11.7					8.4
New York Proton Center
Long term notes receivable and available for sale securities [Line Items]
Accounts receivable, includes unbilled accounts receivable			24.1					26.6
MPTC loans (1)
Long term notes receivable and available for sale securities [Line Items]
Accounts receivable, includes unbilled accounts receivable			0.5					0.3
Cash received				$ 6.0
C P T C
Long term notes receivable and available for sale securities [Line Items]
Accounts receivable, includes unbilled accounts receivable			1.8					1.5
Impairment charges										$ 51.4
Liabilities assumed by lenders	112.0
Varian Medical Systems, Inc. | C P T C
Long term notes receivable and available for sale securities [Line Items]
Additional financing														$ 7.3
Loans Receivable | Proton Center, Munich
Long term notes receivable and available for sale securities [Line Items]
Payments to acquire outstanding senior debt						$ 24.5	€ 21.5
Loans Receivable | NYPC loan (1)
Long term notes receivable and available for sale securities [Line Items]
Maximum lending commitment		$ 91.5
Loans Receivable | C P T C
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	$ 112.0
Revolving Loan | C P T C
Long term notes receivable and available for sale securities [Line Items]
Loans			3.7					4.2
Senior First Lien Loan | NYPC loan (1)
Long term notes receivable and available for sale securities [Line Items]
Maximum lending commitment															$ 73.0
Senior Subordinated Loans | NYPC loan (1)
Long term notes receivable and available for sale securities [Line Items]
Interest rate (as a percent)		13.50%
Maximum lending commitment		$ 18.5
Term (in years)		6 years 6 months
Term Loan | C P T C
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable			44.0					$ 44.0
Term Loan | C P T C
Long term notes receivable and available for sale securities [Line Items]
Credit facility term (in years)	3 years
Term Loan | Varian Medical Systems, Inc. | C P T C
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	$ 53.5
Number of Tranches | tranche	4
Paid-in-kind interest receivable (percent)								7.50%
Term Loan | Varian Medical Systems, Inc. | C P T C | Tranche A loan
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	$ 2.0
Interest receivable, default rate (percent)								9.50%
Term Loan | Varian Medical Systems, Inc. | C P T C | Tranche B loan
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	7.2
Paid-in-kind interest receivable (percent)								10.00%
Interest receivable, default rate (percent)								12.00%
Term Loan | Varian Medical Systems, Inc. | C P T C | Tranche C Loans
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	15.6
Interest receivable, default rate (percent)								9.50%
Term Loan | Varian Medical Systems, Inc. | C P T C | Tranche D Loans
Long term notes receivable and available for sale securities [Line Items]
Modified former loans receivable	$ 28.7
Interest receivable, default rate (percent)								9.50%
Available-for-sale Securities | Georgia Proton Therapy Center
Long term notes receivable and available for sale securities [Line Items]
Payments to acquire available-for-sale securities											$ 16.1
Interest rate (as a percent)							8.00%
Securities sold			8.5
Sale of available-for-sale securities			$ 8.3
Available-for-sale Securities | MPTC Series B-1 Bonds [Member]
Long term notes receivable and available for sale securities [Line Items]
Interest rate (as a percent)								7.50%
Fair Value				25.0
Available-for-sale Securities | MPTC Series B-2 Bonds [Member]
Long term notes receivable and available for sale securities [Line Items]
Interest rate (as a percent)								8.50%
Fair Value				$ 22.9
Forecast | Georgia Proton Therapy Center
Long term notes receivable and available for sale securities [Line Items]
Fair Value												$ 11.3
Forecast | Available-for-sale Securities | MPTC Series B-1 Bonds [Member]
Long term notes receivable and available for sale securities [Line Items]
Fair Value													$ 32.0
Forecast | Available-for-sale Securities | MPTC Series B-2 Bonds [Member]
Long term notes receivable and available for sale securities [Line Items]
Fair Value													$ 33.9
Revolving Loan | Loans Receivable | C P T C
Long term notes receivable and available for sale securities [Line Items]
Extension to the term (in years)								1 year
Maximum lending commitment								$ 15.0
Credit facility term (in years)								1 year
Revolving Loan | Loans Receivable | Varian Medical Systems, Inc. | C P T C
Long term notes receivable and available for sale securities [Line Items]
Maximum lending commitment								$ 7.2
Other income (expense), net | Varian Medical Systems, Inc. | C P T C
Long term notes receivable and available for sale securities [Line Items]
Loss from equity investments								$ (2.2)