Properties, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Properties
Gross	$ 3,980,813	$ 3,874,715	$ 4,158,616	$ 4,014,336
Operating properties, net	2,737,611	2,671,831
Operating properties, net [Member]
Properties
Less: accumulated depreciation	(786,193)	(706,385)
Operating properties, net	2,737,611	2,671,831
Operating properties, net [Member] | Land [Member]
Properties
Gross	455,680	433,311
Operating properties, net [Member] | Buildings and improvements [Member]
Properties
Gross	$ 3,068,124	$ 2,944,905