Segment Information - Operating Revenues by Segment and Type (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue from External Customer [Line Items]
Operating revenue	$ 176,227	$ 171,727	$ 168,567	$ 169,031	$ 168,704	$ 165,192	$ 163,948	[1]	$ 165,397	$ 685,552	$ 663,241	$ 652,009
Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										169,645	174,063	181,412
Northeast Lehigh and Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										96,804	99,311	103,802
Northeast Other [Member]
Revenue from External Customer [Line Items]
Operating revenue										62,823	70,582	74,660
Central Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										120,316	126,268	125,501
South Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										209,656	222,004	232,071
Metro [Member]
Revenue from External Customer [Line Items]
Operating revenue										33,360	28,578	28,617
United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										4,694	4,408	4,211
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										697,298	725,214	750,274
Discontinued Operations [Member]
Revenue from External Customer [Line Items]
Operating revenue										(12,203)	(62,181)	(98,090)
Corporate [Member]
Revenue from External Customer [Line Items]
Operating revenue										457	208	(175)
Industrial Properties [Member]
Revenue from External Customer [Line Items]
Operating revenue										309,809	302,766	304,618
Industrial Properties [Member] | Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										28,604	29,290	30,515
Industrial Properties [Member] | Northeast Lehigh and Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										94,440	93,193	91,826
Industrial Properties [Member] | Northeast Other [Member]
Revenue from External Customer [Line Items]
Operating revenue										16,473	18,369	20,862
Industrial Properties [Member] | Central Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										67,283	61,370	57,438
Industrial Properties [Member] | South Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										93,346	91,439	95,123
Industrial Properties [Member] | Metro [Member]
Revenue from External Customer [Line Items]
Operating revenue										8,356	7,817	7,591
Industrial Properties [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										1,307	1,288	1,263
Office Buildings [Member]
Revenue from External Customer [Line Items]
Operating revenue										387,489	422,448	445,656
Office Buildings [Member] | Northeast Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										141,041	144,773	150,897
Office Buildings [Member] | Northeast Lehigh and Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										2,364	6,118	11,976
Office Buildings [Member] | Northeast Other [Member]
Revenue from External Customer [Line Items]
Operating revenue										46,350	52,213	53,798
Office Buildings [Member] | Central Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										53,033	64,898	68,063
Office Buildings [Member] | South Region [Member]
Revenue from External Customer [Line Items]
Operating revenue										116,310	130,565	136,948
Office Buildings [Member] | Metro [Member]
Revenue from External Customer [Line Items]
Operating revenue										25,004	20,761	21,026
Office Buildings [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										$ 3,387	$ 3,120	$ 2,948

[1] Includes gain on sale included in discontinued operations of $50.2 million.