Reportable Segments (Reconciliation of Net Income to Net Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net Operating Income	$ 80,858	$ 75,815	$ 241,631	$ 236,097
Net operating income from unconsolidated joint ventures	(6,994)	(6,934)	(21,643)	(23,719)
Revenues	118,706	113,159	352,536	352,073
Reimbursed expenses	(955)	(895)	(2,757)	(2,667)
General and administrative expenses	(3,913)	(7,193)	(18,793)	(21,993)
Interest expense	(9,551)	(7,587)	(29,043)	(25,851)
Depreciation and amortization	(45,068)	(47,622)	(135,836)	(152,546)
Acquisition and transaction costs	0	677	(228)	(1,499)
Gain on extinguishment of debt	93	429	8	2,258
Other expenses	(93)	(423)	(457)	(1,063)
Income from unconsolidated joint ventures	2,252	2,461	10,173	43,362
Gain (loss) on sale of investment properties	(33)	(33)	4,912	119,729
Net income	19,859	12,285	58,014	188,088
Fee Income
Segment Reporting Information [Line Items]
Revenues	2,519	2,597	7,211	6,387
Other
Segment Reporting Information [Line Items]
Revenues	$ 744	$ 993	$ 2,836	$ 9,593