PROTON SOLUTIONS LOANS AND INVESTMENTS - Additional Information (Detail) € in Millions, $ in Millions			1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 06, 2017 USD ($) tranche	Jul. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Jul. 31, 2017 EUR (€)	Jan. 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						$ 60.0			$ 58.8
APTC Securities
Variable Interest Entity [Line Items]
Payments to acquire available-for-sale securities			$ 6.0
Interest rate (as a percent)						8.50%
Unbilled receivables from variable interest entity						$ 3.9			2.1
Fair Value						6.7			6.6
RPTC Securities
Variable Interest Entity [Line Items]
Claims filed against assets of Company in bankruptcy | €					€ 77.0
Accounts receivable, includes unbilled accounts receivable						4.2			4.6
New York Proton Center
Variable Interest Entity [Line Items]
Unbilled receivables from variable interest entity						6.4			6.0
Accounts receivable, includes unbilled accounts receivable						18.0			16.6
MPTC loans (1)
Variable Interest Entity [Line Items]
Accounts receivable, includes unbilled accounts receivable						0.0			0.0
C P T C
Variable Interest Entity [Line Items]
Accounts receivable, includes unbilled accounts receivable						2.9			2.6
Impairment charges							$ 51.4
Liabilities assumed by lenders	$ 112.0
Equity ownership interest (percent)	47.08%
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						32.3			31.8
Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable	$ 112.0
Revolving Loan | C P T C
Variable Interest Entity [Line Items]
Loans						5.3			5.3
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						32.3
Term (in years)		6 years 6 months
Term Loan | C P T C
Variable Interest Entity [Line Items]
Modified former loans receivable						$ 44.0			$ 44.0
Credit facility term (in years)	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-1 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)						7.50%
Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Interest rate (as a percent)						8.50%
Forecast | Available-for-sale Securities | MPTC Series B-1 Bonds
Variable Interest Entity [Line Items]
Fair Value								$ 32.0
Forecast | Available-for-sale Securities | MPTC Series B-2 Bonds
Variable Interest Entity [Line Items]
Fair Value								$ 33.9
Revolving Loan | Loans Receivable | C P T C
Variable Interest Entity [Line Items]
Credit facility term (in years)						1 year
Maximum lending commitment						$ 15.0
Revolving Loan | Loans Receivable | Varian Medical Systems, Inc. | C P T C
Variable Interest Entity [Line Items]
Maximum lending commitment						$ 7.2