Acquisitions, Disposals and Assets Held for Sale Acquisitions (Details) $ in Thousands	1 Months Ended	12 Months Ended
Dec. 31, 2018 USD ($) a	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)
Environmental liabilities (1)	$ 17,898	$ 17,898	$ 2,900
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	$ 5,550	$ 5,550
Potomac Land Bay H [Member]
Area of Land in Acres \| a	4.25	4.25
Potomac Land Bay H [Member] \| Alexandria, VA [Member]
Payments to Acquire Land	$ 23,000
Potomac Land Bay H [Member] \| Land [Member]
Environmental liabilities (1)	$ 15,100	$ 15,100
West Half [Member]
Business Acquisition, Percentage of Voting Interests Acquired	3.10%	3.10%
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	$ 5,000	$ 5,000
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions		100.00%

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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