Summary of Significant Accounting Policies - Impact of new accounting pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Rental income									$ 422,852	$ 407,481	$ 383,333
Other operating									1,647	4,149	5,047
Revenues	$ 107,731	$ 106,953	$ 105,245	$ 104,569	$ 105,309	$ 103,659	$ 102,642	$ 100,021	424,499	411,630	388,471
Interest and other income, net									896	375	389
Income from continuing operations	$ (37,151)	$ 3,165	$ 25,224	$ 31,858	$ 52,580	$ 11,857	$ 12,157	$ 9,163	23,096	85,756	$ 58,836
As Reclassified
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Rental income									416,727	401,989
Other operating									8,011	9,966
Revenues									424,738	411,955
Interest and other income, net									658	50
Income from continuing operations									$ 23,096	$ 85,756