Segment Information - Income Statement (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 161,337,000	$ 165,931,000	$ 318,574,000	$ 337,658,000
Interest expense	(25,406,000)	(21,454,000)	(50,903,000)	(42,944,000)
Development service fee income	978,000	19,824,000	1,842,000	46,176,000
Development service fee expense	(847,000)	(79,808,000)	(1,680,000)	(107,875,000)
Depreciation and amortization expense	(43,032,000)	(40,076,000)	(86,430,000)	(79,582,000)
Impairment charges - real estate assets	0	(26,000,000)	(99,000)	(26,000,000)
Gain on property dispositions	5,462,000	48,584,000	5,957,000	52,742,000
Equity in earnings of unconsolidated joint ventures	2,181,000	7,428,000	9,190,000	14,192,000
General and administrative expense	(9,693,000)	(8,832,000)	(25,298,000)	(23,159,000)
Leasing cost	(3,318,000)	(2,767,000)	(6,526,000)	(5,579,000)
Other operating expense	(3,256,000)	(1,577,000)	(5,524,000)	(4,079,000)
Income taxes	261,000	(911,000)	(571,000)	(1,440,000)
Income from continuing operations	54,096,000	8,178,000	94,318,000	49,395,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Operating Segments
Segment Reporting Information [Line Items]
Revenue	171,628,000	169,483,000	342,005,000	341,573,000
Net operating income	116,934,000	113,682,000	230,315,000	229,257,000
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue	49,000	(291,000)	(178,000)	(389,000)
Interest expense	(25,872,000)	(23,202,000)	(52,077,000)	(46,661,000)
Depreciation and amortization expense	(31,778,000)	(31,932,000)	(65,363,000)	(65,340,000)
Impairment charges - real estate assets	0	(26,000,000)	(99,000)	(26,000,000)
Equity in earnings of unconsolidated joint ventures	2,181,000	7,428,000	9,190,000	14,192,000
General and administrative expense	9,563,000	8,302,000	25,304,000	23,054,000
Leasing cost	(3,318,000)	(2,767,000)	(6,526,000)	(5,579,000)
Other operating expense	(3,256,000)	(1,577,000)	(5,524,000)	(4,079,000)
Discontinued operations excluding (loss) gain on property dispositions	580,000	(12,952,000)	(6,034,000)	(25,448,000)
Income taxes	(5,000)	(6,000)	5,000	(18,000)
Other	2,600,000	5,194,000	9,626,000	11,046,000
Development service fee income | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	978,000	19,824,000	1,842,000	46,176,000
Discontinued operations | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenue	(11,318,000)	(23,085,000)	(25,095,000)	(49,702,000)
Carolinas/Richmond | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	20,666,000	19,513,000	40,752,000	39,306,000
Net operating income	14,836,000	13,930,000	29,281,000	27,820,000
Chicago/Minneapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	17,404,000	16,622,000	34,819,000	33,139,000
Net operating income	10,831,000	9,700,000	21,144,000	19,313,000
Cincinnati/Columbus/Indianapolis | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	4,689,000	4,547,000	9,030,000	9,369,000
Net operating income	3,052,000	3,040,000	5,819,000	6,277,000
Dallas | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	5,782,000	4,678,000	11,299,000	8,763,000
Net operating income	3,731,000	2,888,000	7,268,000	5,353,000
Florida | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	16,064,000	15,862,000	32,347,000	31,270,000
Net operating income	10,752,000	10,528,000	22,161,000	21,096,000
Houston | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	16,638,000	16,091,000	32,729,000	31,954,000
Net operating income	9,394,000	8,756,000	18,193,000	18,066,000
Lehigh/Central PA | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	39,503,000	38,525,000	79,710,000	76,610,000
Net operating income	28,278,000	27,576,000	56,818,000	54,604,000
Philadelphia
Segment Reporting Information [Line Items]
Impairment charges - real estate assets		(26,000,000.0)		(26,000,000)
Philadelphia | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	10,925,000	10,717,000	22,263,000	23,061,000
Net operating income	8,694,000	7,921,000	17,219,000	16,826,000
Southern California | Operating Segments | North America
Segment Reporting Information [Line Items]
Revenue	10,600,000	7,516,000	20,150,000	14,311,000
Net operating income	7,248,000	5,765,000	14,680,000	10,848,000
United Kingdom | Operating Segments | United Kingdom
Segment Reporting Information [Line Items]
Revenue	5,959,000	3,783,000	11,957,000	8,056,000
Net operating income	4,942,000	2,311,000	8,237,000	5,495,000
Other | Operating Segments
Segment Reporting Information [Line Items]
Revenue	23,398,000	31,629,000	46,949,000	65,734,000
Net operating income	$ 15,176,000	$ 21,267,000	$ 29,495,000	$ 43,559,000