Summary of Significant Accounting Policies - Impact of new accounting pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Rental income									$ 442,397	$ 416,978	$ 402,205
Other operating									7,992	7,759	9,750
Revenues	$ 113,168	$ 113,462	$ 111,634	$ 112,124	$ 107,749	$ 107,025	$ 105,318	$ 104,644	450,389	424,737	411,955
Interest and other income, net									537	658	51
Income from continuing operations									$ 69,771	23,092	85,571
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Rental income										422,852	407,481
Other operating										1,647	4,149
Revenues										424,499	411,630
Interest and other income, net										896	375
Income from continuing operations										$ 23,092	$ 85,571
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, discount rate (percent)	2.90%								2.90%
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, discount rate (percent)	6.20%								6.20%
Subsequent Event [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Present value of lease payments												$ 90,000
Lease liability												90,000
Subsequent Event [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Present value of lease payments												100,000
Lease liability												$ 100,000