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