Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($) revenue_type	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) revenue_type	Sep. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 2,059	$ 2,010	$ 5,987	$ 5,973
Number of major types of revenue \| revenue_type	3		3
Parking Income
Disaggregation of Revenue [Line Items]
Revenue	$ 1,935	1,752	$ 5,538	5,197
Rental Lease Guaranty
Disaggregation of Revenue [Line Items]
Revenue	0	168	128	488
Management Fee Income
Disaggregation of Revenue [Line Items]
Revenue	69	68	201	205
Miscellaneous
Disaggregation of Revenue [Line Items]
Revenue	$ 55	$ 22	$ 120	$ 83

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration