Segment Information - Income Statement (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenue	$ 157,750,000	$ 172,713,000
Interest expense	(25,519,000)	(22,670,000)
Depreciation and amortization expense	(43,484,000)	(39,659,000)
Gain on property dispositions	741,000	4,158,000
Equity in earnings of unconsolidated joint ventures	7,009,000	6,764,000
General and administrative expense	(15,607,000)	(14,335,000)
Leasing cost	3,208,000	2,812,000
Other operating expense	2,268,000	2,502,000
Income taxes	(832,000)	(528,000)
Income from continuing operations	40,962,000	40,718,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	0	0
Operating Segments
Segment Reporting Information [Line Items]
Revenue	170,380,000	172,088,000
Net operating income	112,272,000	118,154,000
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue	(230,000)	(96,000)
Interest expense	(26,205,000)	(23,459,000)
Depreciation and amortization expense	(33,585,000)	(33,408,000)
Equity in earnings of unconsolidated joint ventures	7,009,000	6,764,000
General and administrative expense	(13,158,000)	(10,725,000)
Leasing cost	(3,208,000)	(2,812,000)
Other operating expense	(2,268,000)	(2,502,000)
Discontinued operations excluding (loss) gain on property dispositions	(6,119,000)	(12,995,000)
Income taxes	(242,000)	(12,000)
Other	5,725,000	(2,445,000)
Development service fee income | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	864,000	26,352,000
Discontinued operations | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	(13,264,000)	(25,631,000)
Carolinas/Richmond | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	20,086,000	19,793,000
Net operating income	14,332,000	14,047,000
Chicago/Minneapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	17,415,000	16,517,000
Net operating income	10,335,000	9,901,000
Cincinnati/Columbus/Indianapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	4,341,000	4,822,000
Net operating income	2,770,000	3,268,000
Dallas | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	5,517,000	4,085,000
Net operating income	3,390,000	2,473,000
Florida | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	16,283,000	15,408,000
Net operating income	11,436,000	10,819,000
Houston | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	16,091,000	15,862,000
Net operating income	8,865,000	9,597,000
Lehigh/Central PA | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	40,208,000	38,085,000
Net operating income	28,427,000	27,767,000
Philadelphia | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	11,338,000	12,345,000
Net operating income	8,075,000	10,154,000
Southern California | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	9,551,000	6,795,000
Net operating income	7,447,000	5,228,000
United Kingdom | Operating Segments | United Kingdom
Segment Reporting Information [Line Items]
Revenue	5,998,000	4,273,000
Net operating income	2,981,000	2,429,000
Other | Operating Segments
Segment Reporting Information [Line Items]
Revenue	23,552,000	34,103,000
Net operating income	$ 14,214,000	$ 22,471,000