Segment Information (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 Reportable_Segments	Dec. 31, 2010	Dec. 31, 2009
Real estate investments, at cost:
Operating properties	$ 4,793,080						$ 4,834,111						$ 4,793,080	$ 4,834,111	$ 4,512,618
Construction-in-progress	25,083						33,322						25,083	33,322	271,962
Land inventory	109,008						110,055						109,008	110,055	97,368
Total revenue	146,609	144,308	144,094	[1]	146,794	[2]	146,084	140,179	134,191	[1]	139,502	[2]	581,805	559,956	567,920
Property operating expenses, real estate taxes and third party management expenses													233,811	227,341	227,160
Net operating income													347,994	332,615	340,760
Number of reportable segments													7
Pennsylvania Suburbs [Member]
Real estate investments, at cost:
Operating properties	1,218,071						1,199,957						1,218,071	1,199,957	1,219,391
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													158,248	152,380	157,616
Property operating expenses, real estate taxes and third party management expenses													58,440	59,065	56,297
Net operating income													99,808	93,315	101,319
Philadelphia CBD [Member]
Real estate investments, at cost:
Operating properties	953,870						911,354						953,870	911,354	490,602
Construction-in-progress							0							0
Land inventory							0							0
Total revenue													125,366	92,110	73,534
Property operating expenses, real estate taxes and third party management expenses													48,792	37,896	31,479
Net operating income													76,574	54,214	42,055
Metropolitan, D.C. [Member]
Real estate investments, at cost:
Operating properties	1,255,803						1,359,776						1,255,803	1,359,776	1,372,213
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													128,456	137,923	140,438
Property operating expenses, real estate taxes and third party management expenses													47,786	48,318	52,899
Net operating income													80,670	89,605	87,539
New Jersey/ Delaware [Member]
Real estate investments, at cost:
Operating properties	545,657						568,413						545,657	568,413	605,181
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													81,629	87,711	98,344
Property operating expenses, real estate taxes and third party management expenses													40,500	44,137	45,421
Net operating income													41,129	43,574	52,923
Richmond, Virginia [Member]
Real estate investments, at cost:
Operating properties	307,698						294,406						307,698	294,406	301,474
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													36,789	36,032	36,201
Property operating expenses, real estate taxes and third party management expenses													14,727	14,010	13,871
Net operating income													22,062	22,022	22,330
Austin, Texas [Member]
Real estate investments, at cost:
Operating properties	257,694						254,019						257,694	254,019	268,806
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													30,895	32,049	31,311
Property operating expenses, real estate taxes and third party management expenses													13,048	13,401	14,351
Net operating income													17,847	18,648	16,960
California [Member]
Real estate investments, at cost:
Operating properties	254,287						246,186						254,287	246,186	254,951
Construction-in-progress							0							0	0
Land inventory							0							0	0
Total revenue													21,670	22,551	29,283
Property operating expenses, real estate taxes and third party management expenses													11,507	11,852	14,735
Net operating income													10,163	10,699	14,548
Corporate [Member]
Real estate investments, at cost:
Operating properties	0						0						0	0	0
Construction-in-progress	25,083						33,322						25,083	33,322	271,962
Land inventory	109,008						110,055						109,008	110,055	97,368
Total revenue													(1,248)	(800)	1,193
Property operating expenses, real estate taxes and third party management expenses													(989)	(1,338)	(1,893)
Net operating income													$ (259)	$ 538	$ 3,086

[1] The consolidated statement of operations for the second quarter of 2011 also contained an out of period depreciation and amortization expense adjustment of $4.7 million relating to intangible assets representing tenant relationships and in-place leases that should have been written off in prior periods. This resulted in the overstatement of depreciation and amortization expense by $4.7 million million in the current year. During the year ended December 31, 2010, depreciation and amortization expense was overstated by $1.7 million and was understated by $1.4 million, $1.8 million, $1.7 million and $1.5 million during the years ended December 31, 2009, 2008, 2007, and 2006, respectively. As management believes that this error was not material to prior years' consolidated financial statements and that the impact of recording the error in the current year is not material to the Company's consolidated financial statements, the Company recorded the related adjustment during the current year.

[2] During the first quarter of 2011, the Company recorded additional income of $0.5 million related to electricity charges in prior years that were under-billed to a certain tenant. This resulted in the overstatement of total revenue by $0.5 million during the current year and in the understatement of total revenue by $0.3 million and $0.2 million for the years ended December 31, 2009 and 2008, respectively. As management believes that this error was not material to prior years' consolidated financial statements and that the impact of recording the error in the current year is not material to the Company's consolidated financial statements, the Company recorded the related adjustment in the current year.