Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013
Real Estate Investments At Cost [Abstract]
Net gain on sale of interests in real estate	$ (1,187)		$ (5,304)
Operating properties	4,701,289			4,669,289
Construction-in-progress	90,140			74,174
Land inventory	96,427			93,351
Investment in real estate ventures, at equity	180,237			180,512
Total revenue	152,114		139,554
Property operating expenses	(61,974)		(55,069)
Gross Profit	90,140		84,485
Equity in income of real estate ventures	242		1,535
Interest Expense, Debt	31,844		30,914
Amortization of deferred financing costs	1,189		1,161
Interest Expense, Other	272		218
Depreciation and amortization	52,570		49,476
General and administrative expenses	8,181		6,551
Loss on early extinguishment of debt	0		(3)
Interest income	385		58
Gain on sale of undepreciated real estate	1,187		0
Income (Loss) from Continuing Operations, Basic	(2,237)		(2,245)
Income (Loss) from Discontinued Operations	(8)		6,164
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	(2,245)		3,919
Pennsylvania [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,209,328			1,199,105
Investment in real estate ventures, at equity	17,379			17,272
Total revenue	40,574		37,916
Property operating expenses	(14,687)		(14,436)
Gross Profit	25,887		23,480
Equity in income of real estate ventures	32		31
Philadelphia CBD [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,318,170			1,300,666
Investment in real estate ventures, at equity	20,131			19,975
Total revenue	49,414		35,925
Property operating expenses	(18,443)		(13,534)
Gross Profit	30,971		22,391
Equity in income of real estate ventures	(21)		566
Metropolitan Dc [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,219,542			1,214,965
Investment in real estate ventures, at equity	59,847			59,905
Total revenue	30,690		27,785
Property operating expenses	(11,957)		(10,763)
Gross Profit	18,733		17,022
Equity in income of real estate ventures	67		16
New Jersey/ Delaware [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	414,353			414,716
Investment in real estate ventures, at equity	0			0
Total revenue	15,625		14,919
Property operating expenses	(8,692)		(7,031)
Gross Profit	6,933		7,888
Equity in income of real estate ventures	18		908
Richmond, Virginia [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	310,465			310,397
Investment in real estate ventures, at equity	1,355			1,400
Total revenue	8,867		8,765
Property operating expenses	(4,340)		(3,683)
Gross Profit	4,527		5,082
Equity in income of real estate ventures	(45)		(8)
Austin, Texas [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	36,480			36,856
Investment in real estate ventures, at equity	81,525	[1]		81,960
Total revenue	2,081		9,257
Property operating expenses	(1,176)		(3,958)
Gross Profit	905		5,299
Equity in income of real estate ventures	191		22
California
Real Estate Investments At Cost [Abstract]
Operating properties	192,951			192,584
Investment in real estate ventures, at equity	0			0
Total revenue	4,602		4,605
Property operating expenses	(2,409)		(2,160)
Gross Profit	2,193		2,445
Equity in income of real estate ventures	0		0
Corporate [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	0			0
Construction-in-progress	90,140			74,174
Land inventory	96,427			93,351
Investment in real estate ventures, at equity	0			0
Total revenue	261		382
Property operating expenses	(270)		496
Gross Profit	(9)		878
Equity in income of real estate ventures	0		0
Austin Joint Venture [Member]
Real Estate Investments At Cost [Abstract]
Investment in real estate ventures, at equity	$ (1,200)

[1] (a) Does not include the negative investment balance of one real estate venture totaling $1.2 million as of March 31, 2014, which was included in other liabilities.