Segment Information (Summary of net income and EBITDA reconciliation by segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Information
Total revenues	$ 626,039	$ 621,708	$ 1,246,887	$ 1,234,745
Total expenses	442,643	434,894	907,024	1,048,211
Operating income	183,396	186,814	339,863	186,534
Income (loss) from partially owned entities	46,276	642	47,721	(3,598)
Income from real estate fund investments	4,391	16,389	4,659	27,673
Interest and other investment income, net	9,307	10,236	18,535	13,754
Interest and debt expense	(96,797)	(105,576)	(191,082)	(206,065)
Net gains on disposition of wholly owned and partially owned assets	0	159,511	501	160,225
Income before income taxes	146,573	268,016	220,197	178,523
Income tax benefit (expense)	248	(2,109)	(1,957)	(4,940)
Income from continuing operations	146,821	265,907	218,240	173,583
Income from discontinued operations	663	2,475	3,091	3,191
Net income	147,484	268,382	221,331	176,774
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(7,677)	(13,025)	(14,414)	(22,703)
Net income (loss) attributable to the Operating Partnership	139,807	255,357	206,917	154,071
Interest and debt expense (2)	118,585	127,799	234,912	253,919
Depreciation and amortization (2)	168,248	173,352	339,785	348,163
Income tax expense (benefit)	289	4,704	2,718	7,965
EBITDA	426,929	561,212	784,332	764,118
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(44,580)	(74,383)	(61,708)	(133,739)
NOI	382,349	486,829	722,624	630,379
Operating Segments | New York
Segment Information
Total revenues	436,862	425,770	863,101	836,595
Total expenses	279,835	268,135	560,656	537,730
Operating income	157,027	157,635	302,445	298,865
Income (loss) from partially owned entities	(272)	(1,001)	(2,365)	(4,564)
Income from real estate fund investments	0	0	0	0
Interest and other investment income, net	1,499	1,214	2,971	2,329
Interest and debt expense	(60,335)	(56,395)	(118,322)	(110,981)
Net gains on disposition of wholly owned and partially owned assets		159,511	0	159,511
Income before income taxes	97,919	260,964	184,729	345,160
Income tax benefit (expense)	906	(816)	763	(1,775)
Income from continuing operations	98,825	260,148	185,492	343,385
Income from discontinued operations	0	0	0	0
Net income	98,825	260,148	185,492	343,385
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(2,645)	(3,397)	(5,489)	(6,826)
Net income (loss) attributable to the Operating Partnership	96,180	256,751	180,003	336,559
Interest and debt expense (2)	78,202	71,171	154,125	142,369
Depreciation and amortization (2)	110,449	111,314	223,259	219,717
Income tax expense (benefit)	(869)	889	(642)	1,979
EBITDA	283,962	440,125	556,745	700,624
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(26,741)	(50,045)	(52,159)	(97,555)
NOI	257,221	390,080	504,586	603,069
Operating Segments | Washington DC
Segment Information
Total revenues	118,336	127,468	234,543	255,480
Total expenses	82,317	89,106	166,305	345,671
Operating income	36,019	38,362	68,238	(90,191)
Income (loss) from partially owned entities	255	(2,370)	342	(3,679)
Income from real estate fund investments	0	0	0	0
Interest and other investment income, net	(23)	34	41	92
Interest and debt expense	(12,008)	(19,817)	(23,569)	(35,752)
Net gains on disposition of wholly owned and partially owned assets		0	0	0
Income before income taxes	24,243	16,209	45,052	(129,530)
Income tax benefit (expense)	(362)	(318)	(716)	(582)
Income from continuing operations	23,881	15,891	44,336	(130,112)
Income from discontinued operations	0	0	0	0
Net income	23,881	15,891	44,336	(130,112)
Less net income attributable to noncontrolling interests in consolidated subsidiaries	0	0	0	0
Net income (loss) attributable to the Operating Partnership	23,881	15,891	44,336	(130,112)
Interest and debt expense (2)	13,567	21,926	26,748	40,637
Depreciation and amortization (2)	33,648	37,196	69,141	77,795
Income tax expense (benefit)	353	2,205	720	2,470
EBITDA	71,449	77,218	140,945	(9,210)
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(1,826)	(6,067)	(5,892)	(10,264)
NOI	69,623	71,151	135,053	(19,474)
Operating Segments | Other
Segment Information
Total revenues	70,841	68,470	149,243	142,670
Total expenses	80,491	77,653	180,063	164,810
Operating income	(9,650)	(9,183)	(30,820)	(22,140)
Income (loss) from partially owned entities	46,293	4,013	49,744	4,645
Income from real estate fund investments	4,391	16,389	4,659	27,673
Interest and other investment income, net	7,831	8,988	15,523	11,333
Interest and debt expense	(24,454)	(29,364)	(49,191)	(59,332)
Net gains on disposition of wholly owned and partially owned assets	0	0	501	714
Income before income taxes	24,411	(9,157)	(9,584)	(37,107)
Income tax benefit (expense)	(296)	(975)	(2,004)	(2,583)
Income from continuing operations	24,115	(10,132)	(11,588)	(39,690)
Income from discontinued operations	663	2,475	3,091	3,191
Net income	24,778	(7,657)	(8,497)	(36,499)
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(5,032)	(9,628)	(8,925)	(15,877)
Net income (loss) attributable to the Operating Partnership	19,746	(17,285)	(17,422)	(52,376)
Interest and debt expense (2)	26,816	34,702	54,039	70,913
Depreciation and amortization (2)	24,151	24,842	47,385	50,651
Income tax expense (benefit)	805	1,610	2,640	3,516
EBITDA	71,518	43,869	86,642	72,704
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(16,013)	(18,271)	(3,657)	(25,920)
NOI	$ 55,505	$ 25,598	$ 82,985	$ 46,784