Significant Accounting Policies - Investment in Marketable Securities (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Investment in Marketable Securities
Marketable securities	$ 30,208,000	$ 28,638,000
Equity marketable securities	30,208,000	28,638,000
Fair Value
Investment in Marketable Securities
Marketable securities	30,208,000	28,638,000
Equity marketable securities
Investment in Marketable Securities
Equity marketable securities	0	0
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities	30,208,000	28,638,000
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
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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