Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Operating revenue	$ 206,901,000		$ 197,630,000
Net operating income	51,829,000		59,398,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	(34,670,000)		(39,207,000)
Depreciation and amortization expense	(59,431,000)		(57,589,000)
Impairment - real estate assets	(15,739,000)		0
Gain on property dispositions	2,271,000		0
Equity in earnings of unconsolidated joint ventures	6,906,000		4,159,000
General and administrative expense	(18,802,000)		(18,356,000)
Income taxes	(845,000)		(531,000)
Income from continuing operations	31,862,000		26,258,000
Richmond Hampton Roads [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Impairment - real estate assets	(13,400,000)
Southeastern PA [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Impairment - real estate assets	(2,300,000)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue	0		0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue	206,952,000		202,657,000
Net operating income	127,570,000		122,099,000
Operating Segments [Member] | Carolinas [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	9,844,000		8,316,000
Net operating income	6,617,000		5,742,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	9,793,000		8,570,000
Net operating income	6,743,000		5,163,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	12,324,000		11,867,000
Net operating income	7,221,000		7,053,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	33,554,000		29,623,000
Net operating income	22,971,000		19,240,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	12,672,000		13,750,000
Net operating income	5,985,000		6,619,000
Operating Segments [Member] | Orlando [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	5,264,000		7,995,000
Net operating income	3,571,000		5,492,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	9,982,000		9,408,000
Net operating income	7,022,000		6,721,000
Operating Segments [Member] | Richmond Hampton Roads [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	10,490,000		10,068,000
Net operating income	6,229,000		5,920,000
Operating Segments [Member] | South Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	12,206,000		12,309,000
Net operating income	6,829,000		7,527,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	39,238,000		38,437,000
Net operating income	20,988,000		19,600,000
Operating Segments [Member] | Tampa [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	13,462,000		13,173,000
Net operating income	8,702,000		8,441,000
Operating Segments [Member] | United Kingdom [Member] | European Union [Member]
Segment Reporting Information [Line Items]
Operating revenue	3,725,000		4,102,000
Net operating income	2,516,000		2,850,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue	34,398,000		35,039,000
Net operating income	22,176,000		21,731,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue	0		(4,748,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue	(51,000)		(279,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	(34,670,000)	[1]	(39,764,000)	[1]
Depreciation and amortization expense	(43,514,000)	[1],[2]	(44,536,000)	[1],[2]
Impairment - real estate assets	(15,739,000)		0
Gain on property dispositions	2,271,000		0
Equity in earnings of unconsolidated joint ventures	6,906,000		4,159,000
General and administrative expense	(12,574,000)	[1],[2]	(12,401,000)	[1],[2]
Discontinued operations excluding interest and gain on property dispositions	0		(2,290,000)
Income taxes	(707,000)	[2]	(483,000)	[2]
Other	$ 2,319,000		$ (526,000)

[1] Includes activity on discontinued operations.

[2] Excludes costs which are included in determining segment-level net operating income.