Segment Information (Summary of net income and EBITDA reconciliation by segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Information
Total Revenues	$ 638,260	$ 633,197	$ 621,708	$ 613,037	$ 651,581	$ 627,596	$ 616,288	$ 606,802	$ 2,506,202	$ 2,502,267	$ 2,312,512
Total expenses									1,955,411	1,742,019	1,622,619
Operating income									550,791	760,248	689,893
Our share of net (loss) income									165,389	(12,630)	(59,861)
(Loss) income from real estate fund investments									(23,602)	74,081	163,034
Interest and other investment income, net									29,546	26,978	38,752
Interest and debt expense									(402,674)	(378,025)	(412,755)
Net gain on extinguishment of Skyline properties debt									487,877	0	0
Net gain on disposition of wholly owned and partially owned assets									175,735	251,821	13,568
Income before income taxes									983,062	722,473	432,631
Income tax (expense) benefit									(8,312)	84,695	(9,281)
Income from continuing operations									974,750	807,168	423,350
Income from discontinued operations									7,172	52,262	585,676
Net income									981,922	859,430	1,009,026
Less net income attributable to noncontrolling interests									(21,351)	(55,765)	(96,561)
Net income attributable to the Operating Partnership									960,571	803,665	912,465
Interest and debt expense (2)									507,362	469,843	654,398
Depreciation and amortization (2)									694,214	664,637	685,973
Income tax (benefit) expense (2)									11,838	(85,379)	24,248
EBITDA									2,173,985	1,852,766	2,277,084
New York | Operating Segments
Segment Information
Total Revenues									1,713,374	1,695,925	1,520,845
Total expenses									1,093,587	1,032,015	946,466
Operating income									619,787	663,910	574,379
Our share of net (loss) income									(2,379)	655	20,701
(Loss) income from real estate fund investments									0	0	0
Interest and other investment income, net									5,093	7,722	6,711
Interest and debt expense									(216,685)	(194,278)	(183,427)
Net gain on extinguishment of Skyline properties debt									0
Net gain on disposition of wholly owned and partially owned assets									159,511	142,693	0
Income before income taxes									565,327	620,702	418,364
Income tax (expense) benefit									(5,508)	(4,379)	(4,305)
Income from continuing operations									559,819	616,323	414,059
Income from discontinued operations									0	0	463,163
Net income									559,819	616,323	877,222
Less net income attributable to noncontrolling interests									(13,558)	(13,022)	(8,626)
Net income attributable to the Operating Partnership									546,261	603,301	868,596
Interest and debt expense (2)									280,563	248,724	241,959
Depreciation and amortization (2)									435,961	394,028	324,239
Income tax (benefit) expense (2)									5,911	4,766	4,395
EBITDA									1,268,696	1,250,819	1,439,189
Washington DC | Operating Segments
Segment Information
Total Revenues									518,117	532,812	537,151
Total expenses									528,863	390,921	358,019
Operating income									(10,746)	141,891	179,132
Our share of net (loss) income									(7,227)	(6,020)	(4,767)
(Loss) income from real estate fund investments									0	0	0
Interest and other investment income, net									(2)	(262)	183
Interest and debt expense									(72,434)	(68,727)	(75,395)
Net gain on extinguishment of Skyline properties debt									487,877
Net gain on disposition of wholly owned and partially owned assets									15,302	102,404	0
Income before income taxes									412,770	169,286	99,153
Income tax (expense) benefit									(1,083)	(317)	(242)
Income from continuing operations									411,687	168,969	98,911
Income from discontinued operations									0	0	0
Net income									411,687	168,969	98,911
Less net income attributable to noncontrolling interests									0	0	0
Net income attributable to the Operating Partnership									411,687	168,969	98,911
Interest and debt expense (2)									81,723	80,795	87,778
Depreciation and amortization (2)									158,720	178,021	144,124
Income tax (benefit) expense (2)									2,979	(1,610)	288
EBITDA									655,109	426,175	331,101
Other | Operating Segments
Segment Information
Total Revenues									274,711	273,530	254,516
Total expenses									332,961	319,083	318,134
Operating income									(58,250)	(45,553)	(63,618)
Our share of net (loss) income									174,995	(7,265)	(75,795)
Equity in net income (loss)									199,555	211,967	330,237
(Loss) income from real estate fund investments									(23,602)	74,081	163,034
Interest and other investment income, net									24,455	19,518	31,858
Interest and debt expense									(113,555)	(115,020)	(153,933)
Net gain on extinguishment of Skyline properties debt									0
Net gain on disposition of wholly owned and partially owned assets									922	6,724	13,568
Income before income taxes									4,965	(67,515)	(84,886)
Income tax (expense) benefit									(1,721)	89,391	(4,734)
Income from continuing operations									3,244	21,876	(89,620)
Income from discontinued operations									7,172	52,262	122,513
Net income									10,416	74,138	32,893
Less net income attributable to noncontrolling interests									(7,793)	(42,743)	(87,935)
Net income attributable to the Operating Partnership									2,623	31,395	(55,042)
Interest and debt expense (2)									145,076	140,324	324,661
Depreciation and amortization (2)									99,533	92,588	217,610
Income tax (benefit) expense (2)									2,948	(88,535)	19,565
EBITDA									$ 250,180	$ 175,772	$ 506,794