Reportable Segments (Reconciliation of Net Income to Net Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net Operating Income	$ 80,195	$ 80,115	$ 160,773	$ 160,281
Net operating income from unconsolidated joint ventures	(7,228)	(7,609)	(14,649)	(16,783)
Revenues	116,628	119,035	233,830	238,915
Reimbursed expenses	(860)	(907)	(1,802)	(1,772)
General and administrative expenses	(8,071)	(8,618)	(14,880)	(14,828)
Interest expense	(9,714)	(8,523)	(19,492)	(18,264)
Depreciation and amortization	(45,675)	(50,040)	(90,768)	(104,924)
Acquisition and transaction costs	(137)	(246)	(228)	(2,177)
Gain (loss) on extinguishment of debt	0	1,829	(85)	1,829
Other expenses	(44)	(236)	(364)	(612)
Income from unconsolidated joint ventures	5,036	40,320	7,921	40,901
Gain on sale of investment properties	5,317	119,832	4,945	119,761
Net income	21,749	170,945	38,155	175,803
Fee Income
Segment Reporting Information [Line Items]
Revenues	1,798	1,854	4,692	3,791
Other
Segment Reporting Information [Line Items]
Revenues	$ 1,132	$ 3,174	$ 2,092	$ 8,600