Mortgages Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Additional Information (Detail) (USD $)	12 Months Ended													12 Months Ended		1 Months Ended	12 Months Ended							0 Months Ended	12 Months Ended																																	12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Minimum	Dec. 31, 2013 Maximum	Dec. 31, 2013 Kentlands Square II and Cranberry Square Loan	Dec. 31, 2013 White Oak Shopping Center	Dec. 31, 2013 Metro Pike Center	Dec. 31, 2013 Northrock	Dec. 31, 2013 Cruse Market Place	Dec. 31, 2013 Beacon Center	Dec. 31, 2013 Northrock Loan	Dec. 31, 2013 Northrock Loan Northrock	Dec. 31, 2013 5.31% due April 2026 Clarendon Center	Sep. 23, 2011 4.30% due October2026 Severna Park Market Place	Dec. 31, 2013 4.30% due October2026 Severna Park Market Place	Dec. 31, 2013 4.53% due November 2026 Kentlands Square II	Dec. 31, 2013 4.70% due December 2026 Cranberry Square	Dec. 31, 2013 5.84% due May 2027 Seven Corners Shopping Center	Dec. 31, 2013 4.04% due April 2028 Hampshire Langley	Dec. 31, 2013 3.51% due June 2028 Beacon Center	Dec. 31, 2013 3.99% due September 2028 Seabreeze Plaza	Oct. 25, 2013 4.88% due September 2032 Park Van Ness	Dec. 31, 2013 4.88% due September 2032 Park Van Ness	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable White Oak Shopping Center	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable Metro Pike Center bank loan	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 5.31% due April 2026		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 5.31% due April 2026	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 4.30% due October2026		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 4.30% due October2026	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 4.53% due November 2026		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 4.53% due November 2026	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 4.70% due December 2026		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 4.70% due December 2026	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 5.84% due May 2027		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 5.84% due May 2027	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 4.04% due April 2028		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 4.04% due April 2028	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 3.51% due June 2028		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 3.51% due June 2028	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 3.99% due September 2028		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 3.99% due September 2028	Dec. 31, 2013 Fixed Rate Mortgage Notes Payable 4.88% due September 2032		Dec. 31, 2012 Fixed Rate Mortgage Notes Payable 4.88% due September 2032	Dec. 31, 2013 Variable Rate Loans Payable		Dec. 31, 2012 Variable Rate Loans Payable	Dec. 31, 2013 Variable Rate Loans Payable Northrock Loan		Dec. 31, 2012 Variable Rate Loans Payable Northrock Loan	Dec. 31, 2013 Variable Rate Loans Payable Metro Pike Center bank loan		Dec. 31, 2012 Variable Rate Loans Payable Metro Pike Center bank loan	Dec. 31, 2013 Debt Covenant	Dec. 31, 2012 Debt Covenant	Dec. 31, 2013 Debt Covenant Minimum	Dec. 31, 2013 Unsecured Revolving Credit Facility	Dec. 31, 2013 Unsecured Revolving Credit Facility LIBOR	Dec. 31, 2013 Unsecured Revolving Credit Facility LIBOR Minimum	Dec. 31, 2013 Unsecured Revolving Credit Facility LIBOR Maximum
Debt Instrument [Line Items]
Fixed rate mortgages	$ 820,068,000		$ 789,776,000																							$ 789,872,000		$ 774,831,000			$ 118,128,000	[1]	$ 120,822,000	$ 36,075,000	[2]	$ 36,986,000	$ 40,974,000	[3]	$ 41,970,000	$ 19,118,000	[4]	$ 19,569,000	$ 70,856,000	[5]	$ 72,233,000	$ 17,718,000	[6]	$ 0	$ 34,391,000	[7]	$ 0	$ 17,895,000	[8]	$ 0	$ 0	[9]	$ 0
Variable rate loans																																																										30,196,000		52,945,000	14,802,000	[10]	14,945,000	15,394,000	[11]	0
Debt outstanding	820,068,000		827,800,000
Debt outstanding with fixed-rate			774,800,000
Debt outstanding with variable-rate			53,000,000
Line of credit facility, maximum borrowing capacity	164,200,000																																																																					175,000,000
Line of credit facility maturity date																																																																						May 20, 2016
Extension in line of credit facility period																																																																						1 year
Letter of credit facility	628,229
Interest rate spread on LIBOR					1.60%	2.50%			1.65%	1.65%																																																1.65%			1.65%			1.65%
Guarantor obligations, maximum exposure, percentage													50.00%
Guarantor obligations, maximum exposure	51,000,000		51,000,000										7,400,000																	7,700,000
Debt instrument maturity term	9 years 9 months 15 days	[12]					6 months								15 years		15 years	15 years	15 years	15 years						10 years 1 month 0 days	[12]																															2 years 2 months 0 days	[12]
Debt instrument amount							60,000,000	10,500,000	15,600,000	15,000,000					125,000,000		38,000,000	43,000,000	20,000,000	73,000,000	18,000,000	35,000,000	18,000,000		71,580,000
Debt instrument, maturity year									2016	2016					2026		2026	2026	2026	2027	2028	2028	2028
Debt instrument fixed interest rate								8.11%							5.31%		4.30%	4.53%	4.70%	5.84%	4.04%	3.51%	3.99%		4.88%
Debt instrument required periodic principal and interest payment									48,000					47,000	753,000		207,000	240,000	113,000	463,200	95,400	203,200	94,900		413,500
Principal amortization period								25 years	25 years	25 years		20 years			25 years	25 years	25 years	25 years	25 years	25 years	25 years	20 years	25 years	25 years	25 years
Debt instrument required final principal payment								18,500,000	14,800,000	14,200,000				14,200,000	70,500,000		20,300,000	23,100,000	10,900,000	42,300,000	9,500,000	11,400,000	9,500,000	39,600,000
Payments of debt financing costs															104,200,000
Number of bridge financing loans							2
Defeased mortgage loan	71,308,000		117,595,000	82,685,000														40,000,000	20,000,000	63,000,000
Proceeds from debt	101,600,000		83,500,000	286,000,000																10,000,000
Debt instrument fixed interest rate																													4.90%
Blended fixed interest rate								7.00%
Line of credit facility, interest rate basis points																																																																							1.60%	1.60%	2.50%
Debt instrument, term									3 years	3 years											15 years	15 years	15 years
Repayment of debt									15,900,000	15,000,000	6,900,000												13,500,000
Existing loan extension period										2 years
Payments of debt extinguishment costs																							497,000
Carrying value of properties collateralizing mortgage notes payable																																																																			907,200,000	916,100,000
Limit in the amount of debt so as to maintain a gross asset value																																																																					503,300,000
Percent of net proceeds from equity offerings																																																																			80.00%
Leverage ratio																																																																			60.00%
Interest coverage ratio																																																																			2.0
Fixed charge coverage ratio																																																																			1.3
Variable interest percent rate																																																																			40.00%
Capitalization of debt issuance cost	$ 3,200,000		$ 2,200,000	$ 1,400,000

[1] The loan is collateralized by Clarendon Center and requires equal monthly principal and interest payments of $753,000 based upon a 25-year amortization schedule and a final payment of $70.5 million at loan maturity. Principal of $2.7 million was amortized during 2013.

[2] The loan is collateralized by Severna Park MarketPlace and requires equal monthly principal and interest payments of $207,000 based upon a 25-year amortization schedule and a final payment of $20.3 million at loan maturity. Principal of $911,000 was amortized during 2013.

[3] The loan is collateralized by Kentlands Square II and requires equal monthly principal and interest payments of $240,000 based upon a 25-year amortization schedule and a final payment of $23.1 million at loan maturity. Principal of $996,000 was amortized during 2013.

[4] The loan is collateralized by Cranberry Square and requires equal monthly principal and interest payments of $113,000 based upon a 25-year amortization schedule and a final payment of $10.9 million at loan maturity. Principal of $451,000 was amortized during 2013.

[5] The loan in the original amount of $73.0 million closed in May 2012, is collateralized by Seven Corners and requires equal monthly principal and interest payments of $463,200 based upon a 25-year amortization schedule and a final payment of $42.3 million at loan maturity. Principal of $1.4 million was amortized during 2013.

[6] The loan is collateralized by Hampshire Langley and requires equal monthly principal and interest payments of $95,400 based upon a 25 -year amortization schedule and a final payment of $9.5 million at loan maturity. Principal of $282,000 was amortized in 2013.

[7] The loan is collateralized by Beacon Center and requires equal monthly principal and interest payments of $203,200 based upon a 20-year amortization schedule and a final payment of $11.4 million at loan maturity. Principal of $609,000 was amortized in 2013.

[8] The loan is collateralized by Seabreeze Plaza and requires equal monthly principal and interest payments of $94,900 based upon a 25-year amortization schedule and a final payment of $9.5 million at loan maturity. Principal of $105,000 was amortized in 2013.

[9] The loan is a $71.6 million construction-to-permanent facility that is collateralized by and will finance a portion of the construction costs of Park Van Ness. During the construction period, interest will be funded by the loan. After conversion to a permanent loan, monthly principal and interest payments totaling $413,500 will be required based upon a 25-year amortization schedule. A final payment of $39.6 million will be due at maturity.

[10] The loan is collateralized by Northrock and requires monthly principal and interest payments of approximately $47,000 and a final payment of $14.2 million at maturity. Principal of $161,000 was amortized during 2013.

[11] The loan is collateralized by Metro Pike Center and requires monthly principal and interest payments of approximately $48,000 and a final payment of $14.8 million at loan maturity. Principal of $206,000 was amortized during 2013.

[12] Interest rate and scheduled maturity data presented as of December 31, 2013. Totals computed using weighted averages.