Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) a item	Mar. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Operating assets independent of the Operating Partnership	$ 0
Useful life of property	40 years
Depreciation expense from operation	$ 26,900,000	$ 25,000,000
Real estate cost capitalized excluding interest cost	3,500,000	2,500,000
Real estate interest cost capitalized incurred	5,400,000	3,100,000
Impairment losses	$ 4,100,000
Number of reporting units | item	1
Term Loan Maturing 2025
Summary Of Significant Accounting Policies [Line Items]
Interest rate	4.75%
Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation	$ 23,700,000	21,400,000
Non-Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation	$ 3,200,000	$ 3,600,000
Land in Manassas, Virginia | Construction in Progress
Summary Of Significant Accounting Policies [Line Items]
Acres of land | a	61
Payments to Acquire Land	$ 24,600,000
Minimum | Real Property
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	20 years
Maximum | Real Property
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	40 years
Maximum | Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	20 years