Significant Accounting Policies - Investment in Marketable Securities (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Investment in Marketable Securities
Marketable securities	$ 29,978,000	$ 28,638,000
Fair Value
Investment in Marketable Securities
Marketable securities	29,978,000	28,638,000
Equity marketable securities
Investment in Marketable Securities
Cost basis	0	0
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities	29,978,000	28,638,000
Cost basis	$ 27,500,000	$ 27,500,000