Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			0 Months Ended			3 Months Ended		0 Months Ended			1 Months Ended		1 Months Ended			3 Months Ended			1 Months Ended		1 Months Ended	0 Months Ended	1 Months Ended	3 Months Ended
	Sep. 30, 2013 property sqft	Sep. 30, 2012	Sep. 30, 2013 property sqft	Sep. 30, 2012	Dec. 31, 2012	Aug. 13, 2013 Acquisition sqft property	Sep. 30, 2013 Acquisition Office property property	Sep. 30, 2013 Acquisition Warehouse property property	Sep. 30, 2013 Sale of properties Office property	Aug. 31, 2013 Chester, VA Acquisition Office property U.S. Government and occupied by the United States Army	Aug. 13, 2013 Chester, VA Acquisition Warehouse property sqft property	Aug. 13, 2013 Bethesda, MD Acquisition Office property sqft property	Aug. 31, 2013 Bethesda, MD Acquisition Office property U.S. Government and the National Institutes of Health	Sep. 30, 2013 Rancho Cordova, California Acquisition Office property sqft	Sep. 30, 2013 Rancho Cordova, California Acquisition Office property State of California and the Department of Consumer Affairs	Oct. 31, 2013 Montgomery, AL Acquisition Office property Subsequent event sqft	Oct. 31, 2013 Montgomery, AL Acquisition Office property U.S. Government and the Social Security Administration Subsequent event	Dec. 31, 2012 Florence, KY Maximum	Sep. 30, 2013 Florence, KY Acquisition Office property	Sep. 30, 2013 Florence, KY Acquisition Office property Level 3 inputs	Dec. 31, 2012 Florence, KY Acquisition Office property Level 3 inputs	Sep. 30, 2013 Fairfax, VA item	Sep. 30, 2013 Fairfax, VA Commonwealth of Virginia and Northern Virginia Community College	Sep. 30, 2013 Fairfax, VA Acquisition Office property sqft property	Feb. 28, 2013 Oklahoma City, OK Sale of properties Office property sqft	Mar. 31, 2013 Tucson, AZ Sale of properties Office property sqft	Sep. 30, 2013 Phoenix, AZ, San Diego, CA and Falls Church, VA Sale of properties Office property sqft property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties owned	81		81
Number of properties owned which were classified as discontinued operations	3		3
Aggregate investment in properties	$ 1,506,891		$ 1,506,891
Percentage of property leased										100.00%			100.00%		100.00%		100.00%					100.00%	51.00%
Number of leases entered	15		37
Expenditures committed on leases	6,324		16,126
Weighted average lease term	7 years 9 months 18 days		9 years 1 month 6 days
Committed but unspent tenant related obligations	14,808		14,808
Rentable square feet of Properties																									185,881	31,051
Net book value of real estate to be sold	1,327,318		1,327,318		1,311,202																				8,069	2,080	25,567
Number of properties acquired or agreed to be acquired						2	1	1			1	1												4			3
Square Feet	245,446		758,491			356,753					228,108	128,645		93,807		49,370								170,940			356,163
Purchase Price						30,803					12,503	18,300
Land						4,827					1,478	3,349
Buildings and Improvements						20,746					9,594	11,152
Acquired Leases						5,622					1,440	4,182
Acquired Lease Obligations						(392)					(9)	(383)
Contract purchase price														20,790		16,050								31,500
Number of tenants																						8
Aggregate sale price of properties sold, excluding closing costs																									16,300	2,189
Gain (loss) on sale of properties																									8,198	(30)
Additional purchase consideration entitled to be paid to the seller, based upon the property's 2013 real estate tax assessment																		1,800
Estimated fair value of additional consideration included in the purchase price																				1,231	273
Acquisition related costs	1,562	763	1,701	1,057															958
Loss on impairment recognized			$ 10,142						$ 10,142