Summary of Significant Accounting Policies (Narrative) (Details)	1 Months Ended	3 Months Ended			6 Months Ended
	Mar. 31, 2018 USD ($) a	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Operating assets independent of the Operating Partnership		$ 0			$ 0
Useful life of property					40 years
Depreciation expense from operation		28,100,000	$ 25,500,000		$ 55,000,000	$ 50,500,000
Real estate cost capitalized excluding interest cost		3,700,000	3,400,000		7,200,000	5,900,000
Real estate interest cost capitalized incurred		6,000,000	3,200,000		11,400,000	6,300,000
Impairment losses		$ 200,000	0		$ 4,300,000	0
Number of reporting units | item					1
Term Loan Maturing 2025
Summary Of Significant Accounting Policies [Line Items]
Interest rate		4.75%			4.75%
Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation		$ 25,000,000	22,000,000		$ 48,600,000	43,400,000
Non-Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation		3,100,000	$ 3,500,000		$ 6,400,000	7,100,000
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
Restructuring Charges [Member]
Summary Of Significant Accounting Policies [Line Items]
Loss on sale of equipment		$ 2,800,000		$ 0		$ 0