Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Real Estate Investments At Cost [Abstract]
Net gain on sale of interests in real estate				$ (2,087)		$ (3,044)
Operating properties	4,689,892			4,689,892			4,669,289
Construction-in-progress	92,713			92,713			74,174
Land inventory	90,266			90,266			93,351
Investment in real estate ventures, at equity	186,042			186,042			180,512
Total revenue	150,500		140,644	302,614		280,198
Property operating expenses	(57,707)		(54,973)	(119,681)		(110,042)
Gross Profit	92,793		85,671	182,933		170,156
Equity in income (loss) of real estate ventures	(489)		1,508	(247)		3,043
Depreciation and amortization	52,587		49,241	105,157		98,717
General and administrative expenses	6,005		7,336	14,186		13,887
Interest Expense, Debt	31,512		30,437	63,356		61,351
Amortization of deferred financing costs	1,197		1,183	2,386		2,344
Interest Expense, Other	316		211	588		429
Loss on early extinguishment of debt	0		(1,113)	0		(1,116)
Gain (Loss) on sale of undepreciated real estate	(3)		0	1,184		0
Income (Loss) from Continuing Operations, Basic	1,245		9,310	(992)		7,065
Income (Loss) from Discontinued Operations	929		(2,130)	921		4,034
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	2,174		7,180	(71)		11,099
Philadelphia CBD [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,321,740			1,321,740			1,300,666
Investment in real estate ventures, at equity	20,689			20,689			19,975
Total revenue	51,420		36,142	100,834		72,067
Property operating expenses	(18,789)		(13,587)	(37,231)		(27,121)
Gross Profit	32,631		22,555	63,603		44,946
Equity in income (loss) of real estate ventures	0		577	(21)		1,143
Pennsylvania [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,228,752			1,228,752			1,199,105
Investment in real estate ventures, at equity	17,544			17,544			17,272
Total revenue	40,743		37,002	81,317		74,918
Property operating expenses	(13,742)		(13,704)	(28,431)		(28,310)
Gross Profit	27,001		23,298	52,886		46,608
Equity in income (loss) of real estate ventures	(391)		558	(359)		588
Metropolitan Dc [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	1,221,305			1,221,305			1,214,965
Investment in real estate ventures, at equity	61,604			61,604			59,905
Total revenue	28,949		28,783	59,639		56,568
Property operating expenses	(10,505)		(10,773)	(22,462)		(21,536)
Gross Profit	18,444		18,010	37,177		35,032
Equity in income (loss) of real estate ventures	(42)		(35)	25		(19)
New Jersey/ Delaware [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	414,754			414,754			414,716
Investment in real estate ventures, at equity	0			0			0
Total revenue	14,841		15,024	30,466		29,944
Property operating expenses	(7,531)		(6,871)	(16,223)		(13,902)
Gross Profit	7,310		8,153	14,243		16,042
Equity in income (loss) of real estate ventures	103		167	122		1,075
Richmond, Virginia [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	310,751			310,751			310,397
Investment in real estate ventures, at equity	1,295			1,295			1,400
Total revenue	8,322		8,954	17,189		17,719
Property operating expenses	(3,566)		(3,599)	(7,906)		(7,282)
Gross Profit	4,756		5,355	9,283		10,437
Equity in income (loss) of real estate ventures	115		243	69		236
Austin, Texas [Member]
Real Estate Investments At Cost [Abstract]
Operating properties	0	[1]		0	[1]		36,856
Investment in real estate ventures, at equity	84,910			84,910			81,960
Total revenue	976		9,872	3,057		19,129
Property operating expenses	(617)		(4,207)	(1,793)		(8,165)
Gross Profit	359		5,665	1,264		10,964
Equity in income (loss) of real estate ventures	(274)		(2)	(83)		20
California
Real Estate Investments At Cost [Abstract]
Operating properties	192,590			192,590			192,584
Total revenue	4,895		4,536	9,497		9,141
Property operating expenses	(2,515)		(2,338)	(4,923)		(4,498)
Gross Profit	2,380		2,198	4,574		4,643
Corporate [Member]
Real Estate Investments At Cost [Abstract]
Construction-in-progress	92,713			92,713			74,174
Land inventory	90,266			90,266			93,351
Total revenue	354		331	615		712
Property operating expenses	(442)		106	(712)		772
Gross Profit	(88)		437	(97)		1,484
Austin Joint Venture [Member]
Real Estate Investments At Cost [Abstract]
Investment in real estate ventures, at equity	$ (1,200)			$ (1,200)

[1] (a)On April 3, 2014, the Company contributed Four Points Centre to an unconsolidated joint venture. See Note 3 for further information on the contribution. Following the contribution, there are no wholly owned properties in the Austin, Texas segment.