Debt Obligations Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Net unamortized premiums/(discounts)	$ (5,817)
Total mortgage indebtedness	437,618	442,974
Total Debt Obligations	2,379,745	2,465,330
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	438,404	443,942
Net unamortized premiums/(discounts)	(786)	(968)
Total mortgage indebtedness	437,618	442,974
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,947,158	2,027,953
Net unamortized premiums/(discounts)	(5,031)	(5,597)
Total unsecured indebtedness	1,942,127	2,022,356
Unsecured Debt [Member] \| Credit Facility
Debt Instrument [Line Items]
Long-term Debt, Gross	0	69,000
Debt Instrument Interest Rates Effective Percentage	LIBOR + 1.50%
Debt Instrument Maturity Dates	Feb-16
Unsecured Debt [Member] \| Three Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	150,000	150,000
Debt Instrument, Interest Rate, Effective Percentage	2.60%
Debt Instrument Maturity Dates	Feb-15
Unsecured Debt [Member] \| Four-Year Term Loan - Variable [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	100,000	100,000
Debt Instrument Interest Rates Effective Percentage	LIBOR + 1.75%
Debt Instrument Maturity Dates	Feb-16
Unsecured Debt [Member] \| Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	200,000	200,000
Debt Instrument, Interest Rate, Effective Percentage	3.62%
Debt Instrument Maturity Dates	Feb-19
Unsecured Debt [Member] \| $250.0M 5.400% Guaranteed Notes due 2014
Debt Instrument [Line Items]
Long-term Debt, Gross	232,199	238,379
Debt Instrument, Interest Rate, Effective Percentage	5.53%
Debt Instrument Maturity Dates	Nov-14
Unsecured Debt [Member] \| $250.0M 7.500% Guaranteed Notes due 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	161,430	166,535
Debt Instrument, Interest Rate, Effective Percentage	7.76%
Debt Instrument Maturity Dates	May-15
Unsecured Debt [Member] \| $250.0M 6.000% Guaranteed Notes due 2016
Debt Instrument [Line Items]
Long-term Debt, Gross	149,919	150,429
Debt Instrument, Interest Rate, Effective Percentage	5.95%
Debt Instrument Maturity Dates	Apr-16
Unsecured Debt [Member] \| $300.0M 5.700% Guaranteed Notes due 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	300,000	300,000
Debt Instrument, Interest Rate, Effective Percentage	5.68%
Debt Instrument Maturity Dates	May-17
Unsecured Debt [Member] \| $325.0M 4.950% Guaranteed Notes due 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	325,000	325,000
Debt Instrument, Interest Rate, Effective Percentage	5.13%
Debt Instrument Maturity Dates	Apr-18
Unsecured Debt [Member] \| $250.0M 3.950% Guaranteed Notes due 2023
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000	250,000
Debt Instrument, Interest Rate, Effective Percentage	4.02%
Debt Instrument Maturity Dates	Feb-23
Unsecured Debt [Member] \| Indenture IA (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	27,062	27,062
Debt Instrument, Interest Rate, Effective Percentage	2.75%
Debt Instrument Maturity Dates	Mar-35
Unsecured Debt [Member] \| Indenture IB (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774	25,774
Debt Instrument, Interest Rate, Effective Percentage	3.30%
Debt Instrument Maturity Dates	Apr-35
Unsecured Debt [Member] \| Indenture II (Preferred Trust II) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774	25,774
Debt Instrument, Interest Rate, Effective Percentage	3.09%
Debt Instrument Maturity Dates	Jul-35
Tysons Corner [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	92,296	93,188
Debt Instrument, Interest Rate, Effective Percentage	5.36%
Debt Instrument Maturity Dates	Aug-15
Two Logan Square [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	88,960	89,340
Debt Instrument, Interest Rate, Effective Percentage	7.57%
Debt Instrument Maturity Dates	Apr-16
Fairview Eleven Tower [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	21,847	22,000
Debt Instrument, Interest Rate, Effective Percentage	4.25%
Debt Instrument Maturity Dates	Jan-17
IRS Philadelphia Campus [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	194,083	197,111
Debt Instrument, Interest Rate, Effective Percentage	7.00%
Debt Instrument Maturity Dates	Sep-30
Cira South Garage [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 41,218	$ 42,303
Debt Instrument, Interest Rate, Effective Percentage	7.12%
Debt Instrument Maturity Dates	Sep-30

Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References+ Details

The amount of debt discount (net of debt premium) that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28567-108399

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References+ Details