Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $)	9 Months Ended
	Sep. 30, 2014		Sep. 16, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Net unamortized premiums/(discounts)	$ (8,080,000)
Total mortgage indebtedness	658,601,000			670,151,000
Total Debt Obligations	2,713,380,000			2,595,381,000
Line of Credit and Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate Terms	Based on the Company's current credit rating, the spread will be 150 basis points under the Credit Facility and 190 basis points under the seven-year term loan.
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	659,512,000			669,762,000
Net unamortized premiums/(discounts)	(911,000)			389,000
Total mortgage indebtedness	658,601,000			670,151,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	2,061,948,000			1,929,703,000
Net unamortized premiums/(discounts)	(7,169,000)			(4,473,000)
Total unsecured indebtedness	2,054,779,000			1,925,230,000
Unsecured Debt [Member] | Three Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0	[1]		150,000,000
Debt Instrument, Interest Rate, Effective Percentage	2.60%
Debt Instrument, Maturity Date	Feb. 01, 2015
Debt Instrument, Repurchased Face Amount			150,000,000
Unsecured Debt [Member] | Four-Year Term Loan - Swapped to fixed [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount			100,000,000
Unsecured Debt [Member] | Four-Year Term Loan - Variable [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0	[1]		100,000,000
Debt Instrument, Interest Rate Terms	LIBOR + 1.75%	[2]
Debt Instrument, Maturity Date	Feb. 01, 2016
Unsecured Debt [Member] | Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	200,000,000			200,000,000
Debt Instrument, Face Amount	200,000,000
Debt Instrument, Interest Rate, Effective Percentage	3.62%
Debt Instrument, Maturity Date	Feb. 01, 2019
Unsecured Debt [Member] | 2014 5.400% Notes
Debt Instrument [Line Items]
Long-term Debt, Gross	143,500,000	[3]		218,549,000
Debt Instrument, Interest Rate, Stated Percentage	5.40%
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Effective Percentage	5.53%
Debt Instrument, Maturity Date	Nov. 01, 2014
Debt Instrument, Repurchased Face Amount			143,500,000
Unsecured Debt [Member] | $250.0M 7.500% Guaranteed Notes due 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	114,919,000	[3]		157,625,000
Debt Instrument, Interest Rate, Stated Percentage	7.50%
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Effective Percentage	7.76%
Debt Instrument, Maturity Date	May 15, 2015
Debt Instrument, Repurchased Face Amount			114,900,000
Unsecured Debt [Member] | $250.0M 6.000% Guranteed Notes due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	149,919,000			149,919,000
Debt Instrument, Interest Rate, Stated Percentage	6.00%
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Effective Percentage	5.95%
Debt Instrument, Maturity Date	Apr. 01, 2016
Unsecured Debt [Member] | $300.0M 5.700% Guaranteed Notes due 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	300,000,000			300,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.70%
Debt Instrument, Face Amount	300,000,000
Debt Instrument, Interest Rate, Effective Percentage	5.68%
Debt Instrument, Maturity Date	May 01, 2017
Unsecured Debt [Member] | $325.0M 4.950% Guaranteed Notes due 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	325,000,000			325,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.95%
Debt Instrument, Face Amount	325,000,000
Debt Instrument, Interest Rate, Effective Percentage	5.13%
Debt Instrument, Maturity Date	Apr. 15, 2018
Unsecured Debt [Member] | $250.0M 3.950% Guaranteed Notes due 2023
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000			250,000,000
Debt Instrument, Interest Rate, Effective Percentage	4.02%
Debt Instrument, Maturity Date	Feb. 15, 2023
Unsecured Debt [Member] | Note 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000	[3]		0
Debt Instrument, Interest Rate, Stated Percentage	4.10%
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Effective Percentage	4.23%
Debt Instrument, Maturity Date	Oct. 01, 2024
Unsecured Debt [Member] | Note 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000	[3]		0
Debt Instrument, Interest Rate, Stated Percentage	4.60%
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Effective Percentage	4.60%
Debt Instrument, Maturity Date	Oct. 01, 2029
Unsecured Debt [Member] | Indenture IA (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	27,062,000			27,062,000
Debt Instrument, Interest Rate, Effective Percentage	2.75%
Debt Instrument, Maturity Date	Mar. 30, 2035
Unsecured Debt [Member] | Indenture IB (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774,000			25,774,000
Debt Instrument, Interest Rate, Effective Percentage	3.30%
Debt Instrument, Maturity Date	Apr. 30, 2035
Unsecured Debt [Member] | Indenture II (Preferred Trust II) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774,000			25,774,000
Debt Instrument, Interest Rate, Effective Percentage	3.09%
Debt Instrument, Maturity Date	Jul. 30, 2035
Tysons Corner [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	89,995,000			91,395,000
Debt Instrument, Interest Rate, Effective Percentage	5.36%	[4]
Debt Instrument, Maturity Date	Aug. 01, 2015
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	123,691,000			125,089,000
Debt Instrument, Interest Rate, Effective Percentage	3.68%	[4]
Debt Instrument, Maturity Date	Jan. 06, 2016
Two Logan Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	87,982,000			88,583,000
Debt Instrument, Interest Rate, Effective Percentage	7.57%
Debt Instrument, Maturity Date	Apr. 01, 2016
Fairview Eleven Tower [Member]
Debt Instrument [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Fairview Eleven Tower [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	21,373,000			21,630,000
Debt Instrument, Interest Rate, Effective Percentage	4.25%	[5]
Debt Instrument, Maturity Date	Jan. 01, 2017
Two Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	112,000,000			112,000,000
Debt Instrument, Interest Rate, Effective Percentage	4.51%	[4]
Debt Instrument, Maturity Date	Apr. 05, 2023
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	186,109,000			190,964,000
Debt Instrument, Interest Rate, Effective Percentage	7.00%
Debt Instrument, Maturity Date	Sep. 10, 2030
Cira South Garage [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 38,362,000			$ 40,101,000
Debt Instrument, Interest Rate, Effective Percentage	7.12%
Debt Instrument, Maturity Date	Sep. 10, 2030

[1] (d)On September 16, 2014, the Company repaid all balances outstanding under its $150.0 million three-year term loan and its $100.0 million four-year term loan prior to the scheduled maturity dates of February 1, 2015 and February 1, 2016, respectively.

[2] (c)London Interbank Offered Rate (“LIBOR”).

[3] (e)On September 16, 2014, the Company closed on an underwritten offering of $250 million 4.10% Guaranteed Notes due 2024 (the "2-24 Notes") and $250.0 million 4.55% Guaranteed Notes due 2029 (the "2029 Notes"). The Company used the net proceeds, together with cash on hand, to redeem a portion of its 5.40% Guaranteed Notes due November 1, 2014 (the "2014 Notes") and its 7.50% Guaranteed Notes due May 15, 2015 (the "2015 Notes"). (See further discussion below).

[4] (a)These loans were assumed upon acquisition of the related properties. The interest rate reflects the market rate at the time of acquisition.

[5] (b)Represents the full debt amount secured by a property owned by a consolidated real estate venture in which the Company holds a 50% interest.