Significant Accounting Policies - Investment in Marketable Securities (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Investment in Marketable Securities
Marketable securities	$ 27,734,000	$ 29,887,000
Equity marketable securities	27,734,000	29,887,000
Fair Value
Investment in Marketable Securities
Marketable securities	27,734,000	29,887,000
Equity marketable securities
Investment in Marketable Securities
Equity marketable securities	0	0
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities	27,734,000	29,887,000
Cost basis of commercial mortgage-backed securities	$ 27,500,000	$ 27,500,000

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Amount of investment in marketable security.

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