Proton Solutions Loans and Investments - NYPC Loan (Details) - USD ($)	Jul. 31, 2015	Sep. 28, 2018	Sep. 29, 2017	Jun. 30, 2016
NYPC loan | Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitment	$ 91,500,000.0
NYPC loan | Senior Subordinated Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitment	$ 18,500,000
Term (in years)	6 years 6 months
Stated rate (as a percent)	13.50%
Deutsche Bank | Senior First Lien Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commitment				$ 73,000,000.0
New York Proton Center [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable		$ 24,100,000	$ 13,300,000
Unbilled accounts receivable			$ 1,300,000