Revenues (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended								9 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Lessor Lease Description [Line Items]
Payment accepted on termination of lease prior to contractual expiration	$ 4,300,000									$ 4,300,000
Total revenues		$ 36,932,000	$ 36,170,000	$ 41,350,000	$ 37,568,000	$ 37,137,000	$ 36,398,000	$ 35,699,000	$ 36,774,000		$ 152,020,000	$ 146,008,000	$ 151,086,000
Expense recoveries											$ 34,100,000	$ 32,100,000	$ 32,700,000
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										$ 3,800,000