Real Estate Investments Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Real Estate and Accumulated Depreciation [Line Items]
Real estate investment property, at cost	$ 2,406,783,000		$ 2,051,936,000
Payments to Acquire Real Estate	281,701,000	[1]	155,881,000	[1]	19,828,000	[1]
Notes Assumed	(78,500,000)		0		0
Credits Received At Settlement	1,300,000		500,000
Total property purchase price	281,700,000		155,900,000		19,900,000
Land Available for Development	9,400,000
Tenant Origination Costs Member
Real Estate and Accumulated Depreciation [Line Items]
Weighted remaining average life, in months	62
Leasing Commissions Absorption Costs Member
Real Estate and Accumulated Depreciation [Line Items]
Weighted remaining average life, in months	51
Net Lease Intangible Assets Member
Real Estate and Accumulated Depreciation [Line Items]
Weighted remaining average life, in months	65
Net Lease Intangible Liabilities Member
Real Estate and Accumulated Depreciation [Line Items]
Weighted remaining average life, in months	62
John Marshall Two And Olney Village Center Member
Real Estate and Accumulated Depreciation [Line Items]
Notes Assumed	76,700,000
Six Fifty North Glebe Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired	11,800,000
Braddock Gateway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired	$ 13,900,000

[1] See note 3 to the consolidated financial statements for the supplemental disclosure of non-cash investing and financing activities, including the assumption of mortgage debt in conjunction with some of our real estate acquisitions.