Real Estate and Accumulated Depreciation (Detail) (USD $) In Thousands, unless otherwise specified						12 Months Ended																																																																			12 Months Ended																							12 Months Ended			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Shopping Centers	Dec. 31, 2013 Shopping Centers Ashburn Village, Ashburn, VA	Dec. 31, 2013 Shopping Centers Ashburn Village, Ashburn, VA Construction One	Dec. 31, 2013 Shopping Centers Ashburn Village, Ashburn, VA Construction One Minimum	Dec. 31, 2013 Shopping Centers Ashburn Village, Ashburn, VA Construction One Maximum	Dec. 31, 2013 Shopping Centers Ashland Square Phase I, Dumfries, VA	Dec. 31, 2013 Shopping Centers Ashland Square Phase I, Dumfries, VA Construction One	Dec. 31, 2013 Shopping Centers Ashland Square Phase I, Dumfries, VA Construction Two	Dec. 31, 2013 Shopping Centers Beacon Center, Alexandria, VA	Dec. 31, 2013 Shopping Centers Beacon Center, Alexandria, VA Minimum	Dec. 31, 2013 Shopping Centers Beacon Center, Alexandria, VA Maximum	Dec. 31, 2013 Shopping Centers Beacon Center, Alexandria, VA Construction One	Dec. 31, 2013 Shopping Centers Beacon Center, Alexandria, VA Construction Two	Dec. 31, 2013 Shopping Centers BJ’s Wholesale Club, Alexandria, VA	Dec. 31, 2013 Shopping Centers Boca Valley Plaza, Boca Raton, FL	Dec. 31, 2013 Shopping Centers Boulevard, Fairfax, VA	Dec. 31, 2013 Shopping Centers Boulevard, Fairfax, VA Construction One	Dec. 31, 2013 Shopping Centers Boulevard, Fairfax, VA Construction Two	Dec. 31, 2013 Shopping Centers Boulevard, Fairfax, VA Construction Three	Dec. 31, 2013 Shopping Centers Briggs Chaney MarketPlace, Silver Spring, MD	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Minimum	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Maximum	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Construction One Minimum	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Construction One Maximum	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Construction Two	Dec. 31, 2013 Shopping Centers Broadlands Village, Ashburn, VA Construction Three	Dec. 31, 2013 Shopping Centers Countryside Marketplace, Sterling, VA	Dec. 31, 2013 Shopping Centers Cranberry Square, Westminster, MD	Dec. 31, 2013 Shopping Centers Cruse MarketPlace, Cumming, GA	Dec. 31, 2013 Shopping Centers Flagship Center, Rockville, MD	Dec. 31, 2013 Shopping Centers French Market, Oklahoma City, OK	Dec. 31, 2013 Shopping Centers French Market, Oklahoma City, OK Construction One	Dec. 31, 2013 Shopping Centers French Market, Oklahoma City, OK Construction Two	Dec. 31, 2013 Shopping Centers Germantown, Germantown, MD	Dec. 31, 2013 Shopping Centers Giant, Milford Mill, MD	Dec. 31, 2013 Shopping Centers The Glen, Woodbridge, VA	Dec. 31, 2013 Shopping Centers The Glen, Woodbridge, VA Construction One	Dec. 31, 2013 Shopping Centers The Glen, Woodbridge, VA Construction Two	Dec. 31, 2013 Shopping Centers Great Eastern, District Heights, MD	Dec. 31, 2013 Shopping Centers Great Eastern, District Heights, MD Construction One	Dec. 31, 2013 Shopping Centers Great Eastern, District Heights, MD Construction Two	Dec. 31, 2013 Shopping Centers Great Falls Center, Great Falls, VA	Dec. 31, 2013 Shopping Centers Hampshire Langley, Takoma, MD	Dec. 31, 2013 Shopping Centers Hunt Club Corners, Apopka, FL	Dec. 31, 2013 Shopping Centers Hunt Club Corners, Apopka, FL Acquisition Date One	Dec. 31, 2013 Shopping Centers Hunt Club Corners, Apopka, FL Acquisition Date Two	Dec. 31, 2013 Shopping Centers Jamestown Place, Altamonte Springs, FL	Dec. 31, 2013 Shopping Centers Kentlands Square I, Gaithersburg, MD	Dec. 31, 2013 Shopping Centers Kentlands Square II, Gaithersburg, MD	Dec. 31, 2013 Shopping Centers Kentlands Place, Gaithersburg, MD	Dec. 31, 2013 Shopping Centers Lansdowne Town Center, Leesburg, VA	Dec. 31, 2013 Shopping Centers Leesburg Pike, Baileys Crossroads, VA	Dec. 31, 2013 Shopping Centers Lumberton Plaza, Lumberton, NJ	Dec. 31, 2013 Shopping Centers Metro Pike Center, Rockville, MD	Dec. 31, 2013 Shopping Centers Shops at Monocacy, Frederick, MD	Dec. 31, 2013 Shopping Centers Shops at Monocacy, Frederick, MD Construction One Minimum	Dec. 31, 2013 Shopping Centers Shops at Monocacy, Frederick, MD Construction One Maximum	Dec. 31, 2013 Shopping Centers Northrock, Warrenton, VA	Dec. 31, 2013 Shopping Centers Olde Forte Village, Ft. Washington, MD	Dec. 31, 2013 Shopping Centers Olde Forte Village, Ft. Washington, MD Construction One Minimum	Dec. 31, 2013 Shopping Centers Olde Forte Village, Ft. Washington, MD Construction One Maximum	Dec. 31, 2013 Shopping Centers Olney, Olney, MD	Dec. 31, 2013 Shopping Centers Orchard Park, Dunwoody, GA	Dec. 31, 2013 Shopping Centers Palm Springs Center, Altamonte Springs, FL	Dec. 31, 2013 Shopping Centers Ravenwood, Baltimore, MD	Dec. 31, 2013 Shopping Centers Ravenwood, Baltimore, MD Construction One	Dec. 31, 2013 Shopping Centers Ravenwood, Baltimore, MD Construction Two	Dec. 31, 2013 Shopping Centers 11503 Rockville Pike/5541 Nicholson Lane, Rockville, MD	Dec. 31, 2013 Shopping Centers 11503 Rockville Pike/5541 Nicholson Lane, Rockville, MD Acquisition Date One	Dec. 31, 2013 Shopping Centers 11503 Rockville Pike/5541 Nicholson Lane, Rockville, MD Acquisition Date Two	Dec. 31, 2013 Shopping Centers 1500 Rockville Pike, Rockville, MD	Dec. 31, 2013 Shopping Centers Seabreeze Plaza, Palm Harbor, FL	Dec. 31, 2013 Shopping Centers Sea Colony (Market Place at), Bethany Beach, DE	Dec. 31, 2013 Shopping Centers Seven Corners, Falls Church, VA	Dec. 31, 2013 Shopping Centers Seven Corners, Falls Church, VA Construction One	Dec. 31, 2013 Shopping Centers Seven Corners, Falls Church, VA Construction Two	Dec. 31, 2013 Shopping Centers Severna Park Marketplace, Severna Park, MD	Dec. 31, 2013 Shopping Centers Shops at Fairfax, Fairfax, VA	Dec. 31, 2013 Shopping Centers Shops at Fairfax, Fairfax, VA Construction One	Dec. 31, 2013 Shopping Centers Shops at Fairfax, Fairfax, VA Construction Two	Dec. 31, 2013 Shopping Centers Smallwood Village Center, Waldorf, MD	Dec. 31, 2013 Shopping Centers Southdale, Glen Burnie, MD	Dec. 31, 2013 Shopping Centers Southdale, Glen Burnie, MD Construction One	Dec. 31, 2013 Shopping Centers Southdale, Glen Burnie, MD Construction Two	Dec. 31, 2013 Shopping Centers Southside Plaza, Richmond, VA	Dec. 31, 2013 Shopping Centers South Dekalb Plaza, Atlanta, GA	Dec. 31, 2013 Shopping Centers Thruway, Winston-Salem, NC	Dec. 31, 2013 Shopping Centers Thruway, Winston-Salem, NC Construction One	Dec. 31, 2013 Shopping Centers Thruway, Winston-Salem, NC Construction Two	Dec. 31, 2013 Shopping Centers Village Center, Centreville, VA	Dec. 31, 2013 Shopping Centers Westview Village, Frederick, MD	Dec. 31, 2013 Shopping Centers White Oak, Silver Spring, MD	Dec. 31, 2013 Shopping Centers White Oak, Silver Spring, MD Construction One	Dec. 31, 2013 Shopping Centers White Oak, Silver Spring, MD Construction Two	Dec. 31, 2013 Shopping Centers Other Buildings / Improvements	Dec. 31, 2013 Mixed-Use Properties	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Minimum	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Maximum	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Construction One	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Construction Two	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Construction Three	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Construction Four	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Construction Five	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Acquisition Date One	Dec. 31, 2013 Mixed-Use Properties Avenel Business Park, Gaithersburg, MD Acquisition Date Two	Dec. 31, 2013 Mixed-Use Properties Clarendon Center, Arlington, VA (1)		Dec. 31, 2013 Mixed-Use Properties Clarendon Center, Arlington, VA (1) Acquisition Date One		Dec. 31, 2013 Mixed-Use Properties Clarendon Center, Arlington, VA (1) Acquisition Date Two		Dec. 31, 2013 Mixed-Use Properties Clarendon Center, Arlington, VA (1) Acquisition Date Three		Dec. 31, 2013 Mixed-Use Properties Crosstown Business Center, Tulsa, OK	Dec. 31, 2013 Mixed-Use Properties 601 Pennsylvania Ave., Washington, DC	Dec. 31, 2013 Mixed-Use Properties Washington Square, Alexandria, VA	Dec. 31, 2013 Mixed-Use Properties Washington Square, Alexandria, VA Construction One	Dec. 31, 2013 Mixed-Use Properties Washington Square, Alexandria, VA Construction Two	Dec. 31, 2013 Development Land	Dec. 31, 2013 Development Land Ashland Square Phase II, Manassas, VA	Dec. 31, 2013 Development Land New Market, New Market, MD	Dec. 31, 2013 Development Land Park Van Ness, Washington, DC	Dec. 31, 2013 Development Land Park Van Ness, Washington, DC Development Date One	Dec. 31, 2013 Development Land Park Van Ness, Washington, DC Development Date Two
Real Estate Properties [Line Items]
Initial Basis	$ 735,254				$ 680,453	$ 11,431				$ 1,178			$ 1,493					$ 22,623	$ 16,720	$ 4,883				$ 27,037	$ 5,316							$ 28,912	$ 31,578	$ 12,226	$ 160	$ 5,781			$ 3,576	$ 998	$ 12,918			$ 4,993			$ 41,750	$ 3,159	$ 12,584			$ 14,055	$ 14,379	$ 72,473	$ 1,425	$ 6,545	$ 2,418	$ 4,400	$ 33,123	$ 9,541			$ 12,686	$ 15,933			$ 1,884	$ 19,377	$ 18,365	$ 1,245			$ 14,861			$ 22,547	$ 24,526	$ 2,920	$ 4,848			$ 63,254	$ 2,708			$ 17,819	$ 3,650			$ 6,728	$ 2,474	$ 4,778			$ 16,502	$ 5,146	$ 6,277				$ 45,179	$ 21,459										$ 12,753	[1]							$ 3,454	$ 5,479	$ 2,034			$ 9,622	$ 5,292	$ 2,088	$ 2,242
Cost Capitalized Subsequent to Acquisition	718,525				387,645	19,433				7,503			18,283					0	1,289	4,746				2,926	25,118							2,753	217	175	9	12,521			742	558	7,528			10,667			2,292	3,327	2,681			744	103	5	7,134	36,016	6,161	11,205	1,316	13,797			14,702	6,499			1,672	413	383	4,107			11,700			135	1,298	26	43,649			55	9,606			7,575	20,754			9,148	3,899	23,108			1,854	22,143	5,351			127	328,906	26,441										185,418	[1]							6,329	59,651	51,067			1,974	1,688	286	0
Basis at Close of Period, Land	357,397				318,943	6,764				1,178			0					22,623	5,735	3,687				9,789	5,300							7,666	6,700	3,920	169	1,118			2,034	422	5,299			3,785			14,766	1,856	4,822			4,455	5,006	20,500	1,425	6,546	1,132	950	26,064	9,260			12,686	5,409			0	7,751	5,739	703			22,113			18,723	8,665	1,146	4,913			12,700	992			6,402	0			1,878	703	5,496			7,851	5,153	4,649				26,858	3,756										16,287	[1]							604	5,667	544			11,596	6,980	2,374	2,242
Basis at Close of Period, Building and Improvements	1,100,221				747,334	24,100				7,503			18,682					0	12,274	5,942				20,174	25,134							23,999	25,095	8,481	0	17,184			2,284	1,134	15,147			11,875			29,276	4,630	10,443			10,344	9,476	51,978	7,134	36,015	7,447	14,655	8,375	14,078			14,702	17,023			3,556	12,039	13,009	4,649			4,448			3,959	17,159	1,800	43,584			50,609	11,322			18,992	23,782			13,998	5,670	22,285			10,505	22,136	6,979			127	347,227	44,144										181,884	[1]							9,179	59,463	52,557			5,660	0	0	5,660
Basis at Close of Period, Leasehold Interests	1,821				1,821	0				0			1,094					0	0	0				0	0							0	0	0	0	0			0	0	0			0			0	0	0			0	0	0	0	0	0	0	0	0			0	0			0	0	0	0			0			0	0	0	0			0	0			0	622			0	0	105			0	0	0				0	0										0	[1]							0	0	0			0	0	0	0
Total	1,459,439	1,466,068	1,417,845	1,224,036	1,068,098	30,864				8,681			19,776					22,623	18,009	9,629				29,963	30,434							31,665	31,795	12,401	169	18,302			4,318	1,556	20,446			15,660			44,042	6,486	15,265			14,799	14,482	72,478	8,559	42,561	8,579	15,605	34,439	23,338			27,388	22,432			3,556	19,790	18,748	5,352			26,561			22,682	25,824	2,946	48,497			63,309	12,314			25,394	24,404			15,876	6,373	27,886			18,356	27,289	11,628			127	374,085	47,900										198,171	[1]							9,783	65,130	53,101			17,256	6,980	2,374	7,902
Accumulated Depreciation	364,663	353,305	326,397	296,786	248,601	10,217				444			12,195					0	3,085	2,153				5,468	7,598							5,826	1,408	2,116		9,257			1,325	1,010	6,716			7,412			4,134	3,232	2,195			2,223	2,716	2,924	2,550	8,968	5,464	11,276	570	3,871			1,795	5,044			3,067	2,000	2,880	2,453			361			818	3,555	270	24,366			2,844	6,375			4,286	20,233			10,661	4,050	12,943			5,393	3,141	5,616			77	116,062	30,861										15,871	[1]							6,599	44,670	18,061			0	0	0	0
Book Value	1,094,776	1,112,763			819,497	20,647				8,237			7,581					22,623	14,924	7,476				24,495	22,836							25,839	30,387	10,285	169	9,045			2,993	546	13,730			8,248			39,908	3,254	13,070			12,576	11,766	69,554	6,009	33,593	3,115	4,329	33,869	19,467			25,593	17,388			489	17,790	15,868	2,899			26,200			21,864	22,269	2,676	24,131			60,465	5,939			21,108	4,171			5,215	2,323	14,943			12,963	24,148	6,012			50	258,023	17,039										182,300	[1]							3,184	20,460	35,040			17,256	6,980	2,374	7,902
Related Debt	$ 820,068	$ 789,776			$ 637,601	$ 30,894				$ 0			$ 34,391					$ 11,795	$ 11,159	$ 6,451				$ 16,419	$ 19,639							$ 16,501	$ 19,118	$ 0	$ 0	$ 0			$ 0	$ 0	$ 9,444			$ 0			$ 15,598	$ 17,718	$ 6,233			$ 8,649	$ 7,853	$ 40,974	$ 0	$ 35,981	$ 17,123	$ 0	$ 15,394	$ 14,610			$ 14,802	$ 11,950			$ 0	$ 10,930	$ 9,921	$ 16,088			$ 0			$ 0	$ 17,894	$ 0	$ 70,856			$ 36,075	$ 9,677			$ 0	$ 0			$ 0	$ 0	$ 42,462			$ 14,849	$ 0	$ 26,153			$ 0	$ 182,467	$ 31,093										$ 118,128	[1]							$ 0	$ 0	$ 33,246			$ 0	$ 0	$ 0	$ 0
Date of Construction							1994	2000	2006		2007	2013				1960	1974				1969	1999	2009					2002	2003	2004	2006				1972		1972	1998	1990	1959		1993	2005		1958	1960		1960					2002		2005	2006	1965	1975			2003	2004	2009		2003	2004	1972				1959	2006								1956	1997			1975	1999			1962	1986	1958	1970		1955	1965	1990	2009		1958	1967						1984	1986	1990	1998	2000			2010	[1]							1974	1986		1952	2000
Date Acquired						1994-03				2004-12			1972-01					2008-03	2004-02	1994-04				2004-04	2002-03							2004-02	2011-09	2004-03	1972-01	1974-03			1993-08	1972-01	1994-06			1972-01			2008-03	1972-01		2006-06	2012-12	2005-11	2002-09	2011-09	2004-01	2002-11	1966-02	1975-12	2010-12	2003-11			2008-01	2007-03			1975-11	2007-07	2005-03	1972-01				2010-10	2012-12	2012-12	2005-11	2008-03	1973-07			2011-09	1975-06			2006-01	1972-01			1972-01	1976-02	1972-05			1993-08	2011-07	1972-01													1984-12	1984-02			1973-07	[1]	1996-01	[1]	2002-04	[1]	1975-10	1973-07	1973-07				2004-12	2005-09		1973-07	2011-02
Buildings and Improvements Depreciable Lives in Years						40 years				20 years				40 years	50 years			0 years	40 years	40 years				40 years		40 years	50 years					40 years	40 years	40 years	0 years	50 years			40 years	40 years	40 years			40 years			40 years	40 years	40 years			40 years	40 years	40 years	50 years	50 years	40 years	40 years	40 years	50 years			50 years	40 years			40 years	40 years	40 years	40 years			40 years			5 years	40 years	40 years	40 years			40 years	50 years			40 years	40 years			40 years	40 years	40 years			40 years	50 years	40 years						35 years	40 years																40 years	35 years	50 years

[1] Includes the North and South Blocks and Residential