Segment Information (Balance Sheet Data) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Balance Sheet Data:
Real estate, at cost	$ 18,339,958	$ 18,090,137	$ 16,822,358	$ 15,392,968
Investments in partially owned entities	1,428,019	1,550,422	1,240,489
Assets	20,814,847	21,143,293	21,157,980
New York | Operating Segments
Balance Sheet Data:
Real estate, at cost	10,787,730	10,577,078	9,732,818
Investments in partially owned entities	1,080,064	1,195,122	1,036,130
Assets	13,312,116	12,257,774	10,706,476
Washington DC | Operating Segments
Balance Sheet Data:
Real estate, at cost	4,152,138	4,544,842	4,383,418
Investments in partially owned entities	94,870	80,708	83,428
Assets	3,645,525	4,517,092	4,281,421
Other | Operating Segments
Balance Sheet Data:
Real estate, at cost	3,400,090	2,968,217	2,706,122
Investments in partially owned entities	253,085	274,592	120,931
Assets	$ 3,857,206	$ 4,368,427	$ 6,170,083