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.