Segment Information (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	3 Months Ended			9 Months Ended
	Sep. 16, 2014	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Real Estate Investments At Cost [Abstract]
Net gain on sale of interests in real estate					$ (5,882)		$ (3,044)
Operating properties		(4,608,895)			(4,608,895)			(4,669,289)
Construction-in-progress		140,456			140,456			74,174
Land inventory		90,842			90,842			93,351
Investment in real estate ventures, at equity		197,539			197,539			180,512
Total revenue		146,558		143,354	449,172		423,552
Property operating expenses		(57,231)		(56,123)	(176,912)		(166,165)
Gross Profit		89,327		87,231	272,260		257,387
Equity in income (loss) of real estate ventures		(486)		714	(733)		3,757
Depreciation and amortization		52,616		50,583	157,773		149,300
General and administrative expenses		5,900		6,436	20,086		20,323
Interest Expense, Debt		31,481		30,338	94,837		91,689
Amortization of deferred financing costs		1,566		1,158	3,952		3,502
Interest Expense, Other		273		264	861		693
Loss on early extinguishment of debt	2,606	(2,606)		(11)	(2,606)		(1,127)
Gain (Loss) on sale of undepreciated real estate		0		(129)	1,184		(129)
Income (Loss) from Continuing Operations, Basic		8,885		11,147	7,893		18,212
Income (Loss) from Discontinued Operations		(3)		(59)	918		3,975
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		8,882		11,088	8,811		22,187
Philadelphia CBD [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(1,327,288)			(1,327,288)			(1,300,666)
Investment in real estate ventures, at equity		20,779			20,779			19,975
Total revenue		49,469		35,891	150,303		107,957
Property operating expenses		(18,168)		(13,704)	(55,399)		(40,825)
Gross Profit		31,301		22,187	94,904		67,132
Equity in income (loss) of real estate ventures		32		405	13		1,548
Pennsylvania [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(1,230,785)			(1,230,785)			(1,199,105)
Investment in real estate ventures, at equity		17,739			17,739			17,272
Total revenue		39,996		39,167	121,313		114,085
Property operating expenses		(13,409)		(13,884)	(41,542)		(42,536)
Gross Profit		26,587		25,283	79,771		71,549
Equity in income (loss) of real estate ventures		(578)		55	(937)		643
Metropolitan Dc [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(1,178,942)			(1,178,942)			(1,214,965)
Investment in real estate ventures, at equity		67,800			67,800			59,905
Total revenue		27,764		29,321	87,403		85,889
Property operating expenses		(10,672)		(10,321)	(33,134)		(31,856)
Gross Profit		17,092		19,000	54,269		54,033
Equity in income (loss) of real estate ventures		(142)		60	(117)		41
New Jersey/ Delaware [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(418,419)			(418,419)			(414,716)
Investment in real estate ventures, at equity		0			0			0
Total revenue		14,975		15,330	45,440		45,273
Property operating expenses		(7,506)		(8,203)	(23,728)		(22,105)
Gross Profit		7,469		7,127	21,712		23,168
Equity in income (loss) of real estate ventures		82		79	204		1,154
Richmond, Virginia [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(311,350)			(311,350)			(310,397)
Investment in real estate ventures, at equity		1,478			1,478			1,400
Total revenue		8,354		8,640	25,543		26,359
Property operating expenses		(3,724)		(3,734)	(11,629)		(11,017)
Gross Profit		4,630		4,906	13,914		15,342
Equity in income (loss) of real estate ventures		184		108	253		344
Austin, Texas [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		0	[1]		0	[1]		(36,856)
Investment in real estate ventures, at equity		89,743			89,743			81,960
Total revenue		843		9,938	3,900		29,067
Property operating expenses		(569)		(3,958)	(2,362)		(12,123)
Gross Profit		274		5,980	1,538		16,944
Equity in income (loss) of real estate ventures		(64)		7	(149)		27
Operating Segments [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(4,659,864)			(4,659,864)			(4,669,289)
Assets Held-for-sale [Member]
Real Estate Investments At Cost [Abstract]
Operating properties		(50,969)	[2]		(50,969)	[2]		0
California
Real Estate Investments At Cost [Abstract]
Operating properties		(193,080)			(193,080)			(192,584)
Total revenue		4,902		4,632	14,399		13,773
Property operating expenses		(2,711)		(2,531)	(7,634)		(7,029)
Gross Profit		2,191		2,101	6,765		6,744
Corporate [Member]
Real Estate Investments At Cost [Abstract]
Construction-in-progress		140,456			140,456			74,174
Land inventory		90,842			90,842			93,351
Total revenue		255		435	871		1,149
Property operating expenses		(472)		212	(1,484)		1,326
Gross Profit		$ (217)		$ 647	$ (613)		$ 2,475
Durham, NC [Member]
Segment Reporting Information [Line Items]
Number of Real Estate Properties		1			1

[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 this contribution, the Company has no wholly-owned properties in the Austin, Texas segment.

[2] (b)On September 30, 2014, the Company was actively marketing for sale of its Valleybrooke Office Park, comprised of five properties located in the Pennsylvania Suburbs segment. As of September 30, 2014 the properties were classified as held for sale on the consolidated balance sheet. The sale is not classified as a significant disposition under the accounting guidance for discontinued operations. Accordingly, the real estate investments remained in the Pennsylvania Suburbs segment as of September 30, 2014.