Segment Information - Income Statement (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 163,169,000	$ 161,286,000	$ 481,743,000	$ 498,944,000
Interest expense	(25,791,000)	(23,133,000)	(76,644,000)	(66,189,000)
Development service fee income	257,000	12,956,000	2,099,000	59,132,000
Development service fee expense	(168,000)	(12,924,000)	(1,848,000)	(120,799,000)
Depreciation and amortization expense	(43,972,000)	(41,004,000)	(130,402,000)	(120,586,000)
Impairment charges - real estate assets			(99,000)	(26,000,000)
Gain on property dispositions	15,168,000	2,002,000	21,125,000	54,705,000
Equity in earnings of unconsolidated joint ventures	1,776,000	6,766,000	10,966,000	20,958,000
General and administrative expense	(10,186,000)	(9,807,000)	(35,483,000)	(32,747,000)
Leasing cost	(3,082,000)	(2,610,000)	(9,608,000)	(8,189,000)
Other operating expense	(3,166,000)	(3,169,000)	(8,690,000)	(7,467,000)
Income taxes	(878,000)	(444,000)	(1,449,000)	(1,884,000)
Income from continuing operations	60,487,000	44,873,000	154,856,000	94,155,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Operating Segments
Segment Reporting Information [Line Items]
Revenue	171,410,000	172,223,000	513,416,000	513,797,000
Net operating income	124,748,000	123,032,000	371,208,000	368,123,000
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue	0	18,000	(179,000)	(372,000)
Interest expense	(26,220,000)	(24,670,000)	(78,297,000)	(71,331,000)
Development service fee income	257,000	12,956,000	2,099,000	59,132,000
Development service fee expense	(168,000)	(12,924,000)	(1,848,000)	(120,799,000)
Depreciation and amortization expense	(32,279,000)	(32,243,000)	(97,642,000)	(97,576,000)
Impairment charges - real estate assets	0	0	(99,000)	(26,000,000)
Gain on property dispositions	15,168,000	2,002,000	21,125,000	54,705,000
Equity in earnings of unconsolidated joint ventures	1,776,000	6,766,000	10,966,000	20,958,000
General and administrative expense	(10,193,000)	(9,814,000)	(35,559,000)	(32,758,000)
Leasing cost	(3,082,000)	(2,610,000)	(9,608,000)	(8,189,000)
Other operating expense	(3,166,000)	(3,169,000)	(8,690,000)	(7,467,000)
Discontinued operations excluding (loss) gain on property dispositions	(4,951,000)	(14,320,000)	(18,553,000)	(39,842,000)
Income taxes	(811,000)	(329,000)	(1,635,000)	(1,539,000)
Other	(592,000)	196,000	1,389,000	(3,262,000)
Development service fee income | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	257,000	12,956,000	2,099,000	59,132,000
Discontinued operations | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	(8,498,000)	(23,911,000)	(33,593,000)	(73,613,000)
Carolinas/Richmond | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	20,377,000	19,417,000	61,129,000	58,723,000
Net operating income	15,638,000	14,620,000	47,020,000	43,795,000
Chicago/Minneapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	17,479,000	17,274,000	52,299,000	50,414,000
Net operating income	10,902,000	10,564,000	33,294,000	31,271,000
Cincinnati/Columbus/Indianapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	4,792,000	4,245,000	13,822,000	13,613,000
Net operating income	3,262,000	2,749,000	9,186,000	9,099,000
Dallas | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	5,953,000	5,537,000	17,252,000	14,300,000
Net operating income	4,035,000	3,666,000	11,848,000	9,274,000
Florida | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	15,871,000	15,847,000	48,218,000	47,117,000
Net operating income	11,628,000	11,461,000	35,097,000	33,717,000
Houston | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	19,103,000	16,561,000	51,831,000	48,514,000
Net operating income	12,377,000	9,676,000	32,159,000	28,714,000
Lehigh/Central PA | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	38,878,000	39,188,000	118,588,000	115,799,000
Net operating income	29,902,000	29,406,000	89,639,000	86,085,000
Philadelphia | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	9,779,000	10,598,000	32,043,000	33,659,000
Net operating income	7,876,000	7,947,000	25,583,000	28,211,000
Southern California | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	10,510,000	7,914,000	30,660,000	22,226,000
Net operating income	8,200,000	6,496,000	23,524,000	17,675,000
United Kingdom | Operating Segments | United Kingdom
Segment Reporting Information [Line Items]
Revenue	5,900,000	3,523,000	17,857,000	11,579,000
Net operating income	5,421,000	3,234,000	16,131,000	10,662,000
Other | Operating Segments
Segment Reporting Information [Line Items]
Revenue	22,768,000	32,119,000	69,717,000	97,853,000
Net operating income	$ 15,507,000	$ 23,213,000	$ 47,727,000	$ 69,620,000