PROTON SOLUTIONS LOANS AND INVESTMENTS - Additional Information (Details) € in Millions, $ in Millions				1 Months Ended				3 Months Ended
	Dec. 06, 2017 USD ($) tranche	Sep. 29, 2017 USD ($)	Jul. 31, 2015 USD ($)	Aug. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Jul. 31, 2017 EUR (€)	Jan. 01, 2021 USD ($)	Jan. 03, 2020 USD ($)	Jan. 01, 2022 USD ($)	Oct. 02, 2020 USD ($)	Apr. 03, 2020 USD ($)	May 01, 2019	Dec. 29, 2017 USD ($)	Jun. 30, 2016 USD ($)
Variable Interest Entity [Line Items]
Fair Value								$ 45.8			$ 44.9
APTC securities
Variable Interest Entity [Line Items]
Payments to acquire available-for-sale securities					$ 6.0
Interest rate (as a percent)								8.50%
Impairment charges on loans									$ 0.9
Trade and unbilled receivables								$ 5.7
Long-term trade receivables, net								7.2			6.9
Fair Value								5.5			5.4
RPTC Securities
Variable Interest Entity [Line Items]
Right to claims against assets | €							€ 77.0
Long-term trade receivables, net								4.1			4.2
New York Proton Center
Variable Interest Entity [Line Items]
Trade and unbilled receivables								1.0			5.0
Long-term trade receivables, net								8.3			20.0
MPTC loans
Variable Interest Entity [Line Items]
Long-term trade receivables, net								0.6			0.6
Cash Received, Outstanding Deferred Payment Arrangement				$ 6.0
C P T C
Variable Interest Entity [Line Items]
Impairment charges on loans		$ 51.4
Long-term trade receivables, net								3.1			3.1
Liabilities assumed by lenders	$ 112.0
Equity ownership interest (percent)	47.08%
Impairment charges								40.5
Value of impaired loan receivable												$ 10.0
CPTC Loans								11.8			11.8
Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Additional financing														$ 7.3
Loans Receivable | Proton Center, Munich
Variable Interest Entity [Line Items]
Payments to acquire outstanding senior debt						$ 24.5	€ 21.5
Loans Receivable | NYPC loan
Variable Interest Entity [Line Items]
Maximum lending commitment			$ 91.5
Long-term								35.7			$ 34.9
Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable	$ 112.0
Senior First Lien Loan | NYPC loan
Variable Interest Entity [Line Items]
Maximum lending commitment															$ 73.0
Senior Subordinated Loans | NYPC loan
Variable Interest Entity [Line Items]
Interest rate (as a percent)			13.50%										10.00%
Maximum lending commitment			$ 18.5
Long-term								$ 35.7
Lending term			6 years 6 months
Term Loan | C P T C
Variable Interest Entity [Line Items]
Credit facility, term	3 years
Term Loan | Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [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
Variable Interest Entity [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
Variable Interest Entity [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
Variable Interest Entity [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
Variable Interest Entity [Line Items]
Modified former loans receivable	$ 28.7
Interest receivable, default rate (percent)								9.50%
Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)				8.50%
Fair Value				$ 22.9
Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)				7.50%
Fair Value				$ 25.0
Available-for-sale Securities | MPTC Bonds
Variable Interest Entity [Line Items]
Impairment charges on loans									$ 16.9
Forecast | Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Fair Value										$ 33.9
Forecast | Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Fair Value										$ 32.0
Revolving Loan | Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Credit facility, term								1 year
Loan facility, maximum lending commitment								$ 15.0
Revolving Loan | Loans Receivable | Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Loan facility, maximum lending commitment								$ 7.2