Segment Information - Income Statement (Details) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenue		$ 190,165,000	$ 165,766,000
Net operating income		59,325,000	55,204,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(22,750,000)	(21,634,000)
Depreciation and amortization expense		(43,686,000)	(43,092,000)
Gain on property dispositions		4,121,000	807,000
Equity in earnings of unconsolidated joint ventures		6,764,000	5,731,000
General and administrative expense		(18,628,000)	(16,993,000)
Expensed pursuit costs		(324,000)	(32,000)
Systems implementation expense		706,000	0
Income taxes		(554,000)	(622,000)
Income from continuing operations		49,392,000	41,348,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		172,088,000	164,079,000
Net operating income		118,154,000	107,207,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(96,000)	(198,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(23,459,000)	(22,343,000)
Depreciation and amortization expense	[1],[2]	(33,408,000)	(33,193,000)
Gain on property dispositions		4,121,000	807,000
Equity in earnings of unconsolidated joint ventures		6,764,000	5,731,000
General and administrative expense	[1],[2]	(15,009,000)	(12,222,000)
Expensed pursuit costs		(324,000)	(32,000)
Systems implementation expense		(706,000)	0
Discontinued operations excluding gain on property dispositions		(4,284,000)	(2,896,000)
Income taxes	[2]	(12,000)	(280,000)
Other		(2,445,000)	(1,431,000)
Discontinued operations | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		(8,179,000)	(9,600,000)
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		26,352,000	11,485,000
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		19,793,000	18,056,000
Net operating income		14,047,000	12,982,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		16,517,000	15,875,000
Net operating income		9,901,000	9,349,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,408,000	14,248,000
Net operating income		10,819,000	9,511,000
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,862,000	14,774,000
Net operating income		9,597,000	6,801,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		38,085,000	41,535,000
Net operating income		27,767,000	29,578,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,345,000	11,436,000
Net operating income		10,154,000	8,759,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,182,000	14,615,000
Net operating income		6,890,000	8,113,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		4,273,000	3,168,000
Net operating income		2,429,000	1,858,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		39,623,000	30,372,000
Net operating income		$ 26,550,000	$ 20,256,000

[1] Includes activity on discontinued operations.

[2] Excludes costs that are included in determining SNOI.