PROTON SOLUTIONS LOANS AND INVESTMENTS - Additional Information (Details) € in Millions				1 Months Ended			9 Months Ended
	Dec. 06, 2017 USD ($) tranche	Sep. 29, 2017 USD ($)	Jul. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Jul. 31, 2017 EUR (€)	Jul. 03, 2020 USD ($)	Jun. 28, 2019 USD ($)	Jan. 01, 2022 USD ($)	Apr. 03, 2020 USD ($)	Sep. 27, 2019 USD ($)	May 01, 2019	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Variable Interest Entity [Line Items]
Fair Value							$ 52,400,000				$ 58,800,000
Impairment charges							$ 49,700,000	$ 50,600,000
APTC securities
Variable Interest Entity [Line Items]
Payments to acquire available-for-sale securities				$ 6,000,000.0
Interest rate (as a percent)							8.50%
Trade and unbilled receivables							$ 5,400,000
Long-term trade receivables, net							6,300,000				2,100,000
Fair Value							5,300,000				6,600,000
RPTC Securities
Variable Interest Entity [Line Items]
Right to claims against assets | €						€ 77.0
Long-term trade receivables, net							4,200,000				4,600,000
New York Proton Center
Variable Interest Entity [Line Items]
Trade and unbilled receivables							5,800,000				6,000,000.0
Long-term trade receivables, net							19,400,000				16,600,000
MPTC loans
Variable Interest Entity [Line Items]
Long-term trade receivables, net							200,000				0
C P T C
Variable Interest Entity [Line Items]
Long-term trade receivables, net							3,100,000				2,600,000
Impairment charges on loans		$ 51,400,000
Liabilities assumed by lenders	$ 112,000,000.0
Equity ownership interest (percent)	47.08%
Impairment charges							40,500,000
Value of impaired loan receivable										$ 10,000,000.0
CPTC Loans							11,800,000				50,500,000
Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Additional financing													$ 7,300,000
Loans Receivable | Proton Center, Munich
Variable Interest Entity [Line Items]
Payments to acquire outstanding senior debt					$ 24,500,000	€ 21.5
Loans Receivable | NYPC loan
Variable Interest Entity [Line Items]
Maximum lending commitment			$ 91,500,000
Long-term							34,000,000.0				31,800,000
Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable	$ 112,000,000.0
Senior First Lien Loan | NYPC loan
Variable Interest Entity [Line Items]
Maximum lending commitment														$ 73,000,000.0
Senior Subordinated Loans | NYPC loan
Variable Interest Entity [Line Items]
Interest rate (as a percent)			13.50%									10.00%
Maximum lending commitment			$ 18,500,000
Lending term			6 years 6 months
Long-term							$ 34,000,000.0
Term Loan | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable											$ 45,200,000
Credit facility, term	3 years
Term Loan | Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable	$ 53,500,000
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	$ 2,000,000.0
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	7,200,000
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	15,600,000
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	$ 28,700,000
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%
Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)							7.50%
Forecast | Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Fair Value									$ 33,900,000
Forecast | Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Fair Value									$ 32,000,000.0
Revolving Loan | Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Credit facility, term							1 year
Loan facility, maximum lending commitment							$ 15,000,000.0
Revolving Loan | Loans Receivable | Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Loan facility, maximum lending commitment							$ 7,200,000