Segment Information (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jan. 02, 2017
Washington DC
Segment Information
Real estate disposition transaction cost			$ 47,000
Operating Segments | New York | 85 Tenth Avenue
Segment Information
Equity method ownership percentage	49.90%		49.90%		49.90%
Operating Segments | Other
Segment Information
Real estate disposition transaction cost	$ 6,211	$ 1,606	$ 13,464	$ 1,858
Operating Segments | Other | Fashion Centre Mall / Washington Tower
Segment Information
Equity method ownership percentage	7.50%		7.50%
Operating Segments | Other | Rosslyn Plaza | Minimum
Segment Information
Equity method ownership percentage	43.70%		43.70%
Operating Segments | Other | Rosslyn Plaza | Maximum
Segment Information
Equity method ownership percentage	50.40%		50.40%