Revenues (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 30, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessor Lease Description [Line Items]
Payment accepted on termination of lease prior to contractual expiration	$ 4,300,000											$ 4,300,000
Total revenues		$ 33,258,000	$ 31,175,000	$ 28,620,000	$ 42,485,000	$ 35,628,000	$ 35,912,000	$ 35,660,000	$ 36,883,000	$ 135,538,000	$ 144,083,000	152,020,000
Expense recoveries										31,100,000	$ 34,400,000	34,100,000
Contract Termination [Member]
Lessor Lease Description [Line Items]
Total revenues												5,400,000
Cash payment reduced by straight line rent receivable												500,000
Accelerated Intangible Lease Liability Amortization [Member] | Contract Termination [Member]
Lessor Lease Description [Line Items]
Total revenues												1,500,000
Other Income [Member] | Contract Termination [Member]
Lessor Lease Description [Line Items]
Total revenues										$ 7,100,000		$ 3,800,000