Segment Information - Operating Revenues by Segment and Type (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenue from External Customer [Line Items]
Operating revenue	$ 169,801	$ 166,247	$ 165,027	$ 166,519	$ 164,530	$ 166,316	$ 161,789	$ 164,366	$ 667,594	$ 657,001	$ 644,696
Segment-level operating revenue									725,214	750,274	759,673
Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									174,063	181,412	185,158
Northeast Lehigh/Central PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									99,311	103,802	101,473
Northeast Other [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									70,582	74,660	82,221
Central Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									126,268	125,501	129,340
South Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									222,004	232,071	235,021
Metro [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									28,578	28,617	22,057
United Kingdom [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									4,408	4,211	4,403
Discontinued Operations [Member]
Revenue from External Customer [Line Items]
Operating revenue									(57,828)	(93,097)	(115,021)
Corporate Elimination [Member]
Revenue from External Customer [Line Items]
Operating revenue									208	(176)	44
Industrial Properties [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									302,766	304,618	307,482
Industrial Properties [Member] | Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									29,290	30,515	33,382
Industrial Properties [Member] | Northeast Lehigh/Central PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									93,193	91,826	88,398
Industrial Properties [Member] | Northeast Other [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									18,369	20,862	21,294
Industrial Properties [Member] | Central Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									61,370	57,438	63,472
Industrial Properties [Member] | South Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									91,439	95,123	96,172
Industrial Properties [Member] | Metro [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									7,817	7,591	3,470
Industrial Properties [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									1,288	1,263	1,294
Office Buildings [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									422,448	445,656	452,191
Office Buildings [Member] | Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									144,773	150,897	151,776
Office Buildings [Member] | Northeast Lehigh/Central PA [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									6,118	11,976	13,075
Office Buildings [Member] | Northeast Other [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									52,213	53,798	60,927
Office Buildings [Member] | Central Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									64,898	68,063	65,868
Office Buildings [Member] | South Region [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									130,565	136,948	138,849
Office Buildings [Member] | Metro [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									20,761	21,026	18,587
Office Buildings [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Segment-level operating revenue									$ 3,120	$ 2,948	$ 3,109