PROTON SOLUTIONS LOANS AND INVESTMENTS - Additional Information (Details) € in Millions			1 Months Ended			3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 06, 2017 USD ($) tranche	Jul. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2017 EUR (€)	Jul. 31, 2017 USD ($)	Apr. 03, 2020 USD ($)	Apr. 03, 2020 USD ($)	Sep. 29, 2017 USD ($)	Jan. 01, 2022 USD ($)	Sep. 27, 2019 USD ($)	May 01, 2019	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)
Variable Interest Entity [Line Items]
Fair Value						$ 60,400,000	$ 60,400,000			$ 58,800,000
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%	8.50%
Trade and unbilled receivables						$ 5,800,000	$ 5,800,000
Long-term trade receivables, net						6,400,000	6,400,000			2,100,000
Fair Value						6,100,000	6,100,000			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	4,200,000			4,600,000
New York Proton Center
Variable Interest Entity [Line Items]
Trade and unbilled receivables						6,500,000	6,500,000			6,000,000.0
Long-term trade receivables, net						19,100,000	19,100,000			16,600,000
MPTC loans
Variable Interest Entity [Line Items]
Long-term trade receivables, net						0	0			0
C P T C
Variable Interest Entity [Line Items]
Long-term trade receivables, net						2,900,000	2,900,000			2,600,000
Impairment charges						40,500,000		$ 51,400,000
Liabilities assumed by lenders	$ 112,000,000.0
Equity ownership interest (percent)	47.08%
Value of impaired loan receivable						10,000,000.0	10,000,000.0
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				€ 21.5	$ 24,500,000
Loans Receivable | NYPC loan
Variable Interest Entity [Line Items]
Maximum lending commitment		$ 91,500,000
Long-term						33,100,000	33,100,000			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						33,100,000	33,100,000
Term Loan | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable						$ 4,700,000	$ 4,700,000			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%	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%	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%
Revolving Loan | C P T C
Variable Interest Entity [Line Items]
Loans funded						$ 5,300,000	$ 5,300,000			$ 5,300,000
Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)						8.50%	8.50%
Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)						7.50%	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