Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended				12 Months Ended				1 Months Ended		1 Months Ended		1 Months Ended		1 Months Ended			1 Months Ended			1 Months Ended			1 Months Ended		1 Months Ended		3 Months Ended	12 Months Ended			12 Months Ended
	Dec. 31, 2013 property sqft state item	Dec. 31, 2012	Dec. 31, 2013 Continuing operations property item	Dec. 31, 2013 Discontinued operations property item	Dec. 31, 2013 Acquisition sqft item property	Dec. 31, 2013 Acquisition Office and Office Warehouse Property state	Dec. 31, 2013 One building Warehouse property property item	Dec. 31, 2013 Seven building Office property item property	Aug. 31, 2013 Chester, VA One building Warehouse property sqft item property	Aug. 31, 2013 Chester, VA One building Warehouse property U.S. Government and occupied by the United States Army	Aug. 31, 2013 Bethesda, MD One building Office property sqft property item	Aug. 31, 2013 Bethesda, MD One building Office property U.S. Government and the National Institutes of Health	Oct. 31, 2013 RanchoCordovaCAMember One building Office property sqft property item	Oct. 31, 2013 RanchoCordovaCAMember One building Office property State of California and the Department of Consumer Affairs	Nov. 30, 2013 Fairfax, VA item	Nov. 30, 2013 Fairfax, VA One building Office property sqft item	Nov. 30, 2013 Fairfax, VA One building Office property U.S. Government	Nov. 30, 2013 Fairfax, VA Four building Office property sqft item property	Nov. 30, 2013 Fairfax, VA Four building Office property Commonwealth of Virginia and Northern Virginia Community College	Nov. 30, 2013 Fairfax, VA Four building Office property U.S. Government and the State Department	Dec. 31, 2013 Montgomery, AL One building Office property sqft item property	Dec. 31, 2013 Montgomery, AL One building Office property U.S. Government and the Social Security Administration	Dec. 31, 2013 Reston, VA Acquisition Office property item	Dec. 31, 2013 Reston, VA Two buildings Office property sqft	Dec. 31, 2013 Reston, VA Two buildings Office property U.S. Government	Feb. 28, 2013 Oklahoma City, OK Sale of properties Office property sqft item	Mar. 31, 2013 Tucson, AZ Sale of properties Office property item sqft	Sep. 30, 2013 Phoenix, AZ, San Diego, CA and Falls Church, VA Sale of properties Office property item	Dec. 31, 2013 Phoenix, AZ, San Diego, CA and Falls Church, VA Sale of properties Office property sqft property item	Dec. 31, 2013 Phoenix, AZ	Jan. 31, 2014 Phoenix, AZ Subsequent event	Dec. 31, 2013 Falls Church, VA Discontinued operations
Real estate properties
Number of properties owned	68		68	3
Number of Buildings	87		87	3
Aggregate investment in properties	$ 1,568,562	$ 1,467,863	$ 1,568,562
Number of leases entered	48
Weighted average lease term	8 years 8 months 12 days
Expenditures committed on leases	25,495
Committed but unspent tenant related obligations	16,557
Future minimum lease payments related to properties excluding discontinued operations, scheduled to be received during the current terms of the existing leases
2014			214,261
2015			198,986
2016			178,076
2017			153,036
2018			128,404
Thereafter			342,036
Total future minimum lease payments			1,214,799
Number of states in which acquired properties located	31					4
Number of properties acquired or agreed to be acquired					5		1	4	1		1		1					1			1								3
Number of buildings acquired or agreed to be acquired					8		1	7	1		1		1			1		4			1		2			1	1		3
Square Feet	1,017,753				670,870				228,108		128,645		93,807			83,130		170,940			49,370			406,388					356,163
Purchase Price					99,518				12,503		18,300		21,190					31,500			16,025
Land					9,292				1,478		3,349		562					2,529			1,374
Buildings and Improvements					70,712				9,594		11,152		16,922					21,386			11,658
Acquired Leases					22,901				1,440		4,182		5,498					8,005			3,776
Acquired Lease Obligations					(3,387)				(9)		(383)		(1,792)					(420)			(783)
Other Assumed Liabilities					(269)													(269)
Percentage of property leased										100.00%		100.00%		100.00%	100.00%		100.00%		51.00%	100.00%		100.00%			100.00%
Number of tenants															8
Contract purchase price																19,775								113,250
Amount of mortgage debt assumed in contract purchase price																14,576								83,000
Deposit made in connection with acquisition																2,000								11,325
Rentable square feet of Properties	10,300,000																									185,881	31,051
Net book value of real estate to be sold	1,380,927	1,311,202																								8,069	2,080		25,604	2,300		12,289
Aggregate sale price of properties sold, excluding closing costs																										16,300	2,189				5,000	15,750
Gain (loss) on sale of properties	8,168																									8,198	(30)
Loss on impairment recognized	$ 10,142	$ 494																										$ 10,142		$ 8,344		$ 1,798
Number of Real Estate Properties on which impairment loss was recognized																												2