Mortgage Notes Payable (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 building		Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Minimum, Buildings or Land, Collateral for Mortgage Notes Payable	1
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum	5.00%
Carrying Amount, Mortgaged Properties	$ 678,900,000		$ 570,500,000
Mortgage notes payable	427,710,000		361,860,000
Mortgage note interest rate, percentage	3.875%
Shady Grove Medical Village II [Member]
Mortgage Loans on Real Estate [Line Items]
Repayment of secured mortgage notes	9,100,000
Columbia Park Road And Dulles Business Park [Member]
Mortgage Loans on Real Estate [Line Items]
Repayment of secured mortgage notes	17,500,000
Increase in sales price for partially offsetting the penalties incurred due to prepayment of mortgage notes	875,000
Prepayment penalties incurred on mortgage notes	1,000,000
Prosperity Medical Center 5.36% [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	31,169,000		31,886,000
Mortgage note, fair value interest rate	5.36%	[1]
Prosperity Medical Center 5.34% [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	11,828,000		12,101,000
Mortgage note, fair value interest rate	5.34%	[1]
Shady Grove Medical Village Ii Member
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	0		9,375,000
Mortgage note, fair value interest rate	6.98%	[1]
Frederick Crossing [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	21,700,000		22,268,000
Mortgage note, fair value interest rate	5.95%	[1]
9707 Medical Center [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	4,780,000	[2]	4,955,000	[2]
Mortgage note, fair value interest rate	5.32%	[1],[2]
Plumtree [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	4,419,000		4,512,000
Mortgage note, fair value interest rate	5.68%	[1]
15005 Shady Grove [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	7,974,000		8,149,000
Mortgage note, fair value interest rate	5.73%	[1]
West Gude [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	30,761,000		31,486,000
Mortgage note, fair value interest rate	5.86%	[1]
Woodholme Medical Center [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	19,954,000		20,285,000
Mortgage note, fair value interest rate	5.29%	[1]
Ashburn Farm Office 5.56% [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	2,438,000		2,556,000
Mortgage note, fair value interest rate	5.56%	[1]
Ashburn Farm Office 5.69% [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	2,159,000		2,285,000
Mortgage note, fair value interest rate	5.69%	[1]
3801 Connecticut [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	35,399,000		81,029,000	[3]
Mortgage note, fair value interest rate	5.71%	[1],[3]
3801 Connecticut, Walker House, Bethesda Hill [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	81,029,000	[3]
2445 M Street [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	95,593,000	[4]	94,339,000	[4]
Mortgage note, fair value interest rate	5.62%	[1],[4]
Kenmore Apartments [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	36,097,000		36,634,000
Mortgage note, fair value interest rate	5.37%	[1]
Olney Village Center Member
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	23,873,000		0
Mortgage note, fair value interest rate	4.94%	[1]
John Marshall II Member
Mortgage Loans on Real Estate [Line Items]
Mortgage notes payable	$ 53,936,000		$ 0
Mortgage note, fair value interest rate	5.79%	[1]

[1] Yield on the assumption/issuance date, including the effects of any premiums, discounts or fair value adjustments on the notes.

[2] The mortgage bears a set interest rate through June 30, 2012, at which time the rate will be reset based on the Moody's Long-term Corporate Bond Yield Average, but never lower than 5.0% per annum. The interest rate will be reset annually thereafter for the duration of the note. Principal and interest are payable monthly until July 1, 2028, at which time all unpaid principal and interest are payable in full. During the first 90 days of 2013, the lender has the option to elect to accelerate the maturity date of the note to July 1, 2013. For the remainder of 2013, we have the right to prepay the note without any prepayment penalties.

[3] Interest only is payable monthly until the maturity date, which can be extended for one year upon which the interest rate is reset on June 1, 2016. At maturity on June 1, 2017, all unpaid principal and interest are payable in full.

[4] Interest only is payable monthly until the maturity date upon which all unpaid principal and interest are payable in full.