Segment Information Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Real Estate Investments At Cost [Abstract]
Rental properties	$ 4,607,890		$ 4,726,169
Construction-in-progress	53,468		48,950
Land inventory	92,776		102,439
Total revenue	140,608	134,539
Cost of Goods and Services Sold	55,496	52,894
Gross Profit	85,112	81,645
Pennsylvania [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	1,179,683		1,178,730
Total revenue	38,187	38,105
Cost of Goods and Services Sold	13,977	13,348
Gross Profit	24,210	24,757
Philadelphia CBD [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	994,939		988,590
Total revenue	35,925	33,149
Cost of Goods and Services Sold	13,534	12,686
Gross Profit	22,391	20,463
Metropolitan Dc [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	1,197,904		1,193,200
Total revenue	27,785	26,437
Cost of Goods and Services Sold	10,763	10,065
Gross Profit	17,022	16,372
New Jersey/ Delaware [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	414,485		546,644
Total revenue	15,602	15,730
Cost of Goods and Services Sold	7,408	7,831
Gross Profit	8,194	7,899
Richmond, Virginia [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	310,621		309,923
Total revenue	8,765	9,034
Cost of Goods and Services Sold	3,683	3,643
Gross Profit	5,082	5,391
Austin, Texas [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	285,540		285,346
Total revenue	9,257	7,495
Cost of Goods and Services Sold	3,958	3,397
Gross Profit	5,299	4,098
CALIFORNIA
Real Estate Investments At Cost [Abstract]
Rental properties	224,718		223,736
Total revenue	5,389	4,971
Cost of Goods and Services Sold	2,395	2,368
Gross Profit	2,994	2,603
Corporate [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	0		0
Construction-in-progress	53,468		48,950
Land inventory	92,776		102,439
Total revenue	(302)	(382)
Cost of Goods and Services Sold	(222)	(444)
Gross Profit	$ (80)	$ 62

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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 2

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2(a),(d))

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The current amount of expenditures for a real estate project that has not yet been completed.

+ References

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Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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- Definition

Carrying amount of land available for development.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.(a),1(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Amount of real estate investment property which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

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The aggregate revenue from real estate operations during the reporting period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

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