Real Estate Acquired - Additional Information (Detail) (USD $)	12 Months Ended				12 Months Ended						1 Months Ended					1 Months Ended				1 Months Ended				1 Months Ended
	Dec. 31, 2013 Property	Dec. 31, 2012 Property	Dec. 31, 2011 Property	Dec. 31, 2012 In-Place Leases	Dec. 31, 2013 Above Market Leases	Dec. 31, 2012 Above Market Leases	Dec. 31, 2011 Above Market Leases	Dec. 31, 2013 Lease Acquisition Costs	Dec. 31, 2012 Lease Acquisition Costs	Dec. 31, 2011 Lease Acquisition Costs	Feb. 28, 2011 4469 Connecticut Avenue	Sep. 30, 2011 Kentlands Square II	Dec. 31, 2013 Kentlands Square II	Dec. 31, 2013 Kentlands Square II In-Place Leases	Dec. 31, 2013 Kentlands Square II Above Market Leases	Sep. 30, 2011 Severna Park Market Place	Dec. 31, 2013 Severna Park Market Place	Dec. 31, 2013 Severna Park Market Place In-Place Leases	Dec. 31, 2013 Severna Park Market Place Above Market Leases	Sep. 30, 2011 Cranberry Square	Dec. 31, 2013 Cranberry Square	Dec. 31, 2013 Cranberry Square In-Place Leases	Dec. 31, 2013 Cranberry Square Above Market Leases	Dec. 31, 2012 1500 Rockville Pike	Dec. 31, 2012 5541 Nicholson Lane	Aug. 31, 2013 Kentlands Pad sqft	Dec. 31, 2013 Hunt Club Pad sqft
Business Acquisition [Line Items]
Year of acquisition											Feb. 28, 2011	Sep. 30, 2011				Sep. 30, 2011				Sep. 30, 2011				Dec. 31, 2012	Dec. 31, 2012	Aug. 31, 2013	Dec. 31, 2013
Aggregate cost	$ 4,300,000	$ 34,100,000	$ 170,100,000								$ 1,600,000	$ 74,500,000				$ 61,000,000				$ 33,000,000				$ 22,400,000	$ 11,700,000	$ 4,300,000	$ 800,000
Name of property											4469 Connecticut Avenue	Kentlands Square II				Severna Park MarketPlace				Cranberry Square				1500 Rockville Pike	5541 Nicholson Lane
Location											Washington, DC	Gaithersburg, Maryland				Severna Park, Maryland				Westminster, Maryland				Rockville, Maryland	Rockville, Maryland	Gaithersburg, Maryland
Property acquisition costs	106,000	1,100,000	2,500,000								74,000	1,100,000				800,000				500,000				600,000	500,000	99,000
Area of real estate property (in square feet)																										7,100	5,500
Number of properties acquired	1	2	4
Purchase price allocated to buildings	2,000,000	3,800,000											51,973,000				50,554,000				24,878,000
Purchase price allocated to land	2,300,000	30,400,000											20,500,000				12,700,000				6,700,000
Purchase price allocated to leases				500,000			28,000							2,697,000	6,000			2,433,000	4,000			1,499,000	18,000
Purchase price allocated to below market lease		700,000	5,500,000										676,000				4,691,000				95,000
Business acquisition, intangible asset estimated useful life		3 years 1 month 6 days	10 years 10 months 24 days				4 years 1 month 6 days
Intangible assets gross carrying amount					1,000,000	1,000,000		21,900,000	21,900,000
Intangible assets, accumulated amortization					974,100	929,100		16,700,000	14,700,000
Amortization expense of intangible assets					45,000	60,000	62,000	2,000,000	2,000,000	1,300,000
Intangible liabilities gross carrying amount	24,800,000	24,800,000
Intangible liabilities, accumulated amortization	10,000,000	8,300,000
Accretion income, intangible liabilities	$ 1,700,000	$ 1,600,000	$ 1,200,000