Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Property rentals									$ 2,157,938	$ 2,099,158	$ 1,989,169
Straight Line Rent Adjustments									41,431	73,007	89,405
Amortization of acquired below-market leases, net									62,442	65,542	70,401
Operating Leases, Income Statement, Lease Revenue, Total									2,261,811	2,237,707	2,148,975
Tenant expense reimbursements									349,420	355,616	351,290
Cleveland Medical Mart development project									154,080	0	0
BMS cleaning fees									61,754	58,053	53,824
Management and leasing fees									20,103	20,117	11,456
Lease termination fees									16,395	14,826	4,886
Other									52,102	54,362	85,160
Total Revenues	741,815	727,343	719,624	726,883	702,836	687,125	674,192	676,528	2,915,665	2,740,681	2,655,591
Operating									1,091,597	1,082,844	1,050,545
Depreciation and amortization									553,811	522,022	519,534
General and administrative									209,981	213,949	230,584
Cleveland Medical Mart development project									145,824	0	0
Tenant buy-outs, impairment losses and other acquisition related costs									58,299	129,458	73,763
Total expenses									2,059,512	1,948,273	1,874,426
Operating income									856,153	792,408	781,165
Income from partially owned entities									120,310	94,062	72,390
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									22,886	(303)	0
Interest and other investment (loss) income, net									148,826	235,315	(116,350)
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(544,015)	(560,052)	(617,768)
Net gain on early extinguishment of debt									0	94,789	(25,915)
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									15,134	81,432	5,641
Income before income taxes									619,294	737,651	99,163
Income tax expense									(24,827)	(22,476)	(20,642)
Income from continuing operations									594,467	715,175	78,521
Income (loss) from discontinued operations									145,533	(7,144)	49,929
Net income									740,000	708,031	128,450
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									(21,786)	(4,920)	2,839
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									(55,912)	(55,228)	(25,120)
Net income (loss) attributable to Common Shareholders									662,302	647,883	106,169
Interest and debt expense (2)									797,920	828,082	826,827
Depreciation and amortization (2)									777,421	729,426	728,815
Income tax (benefit) expense (2)									4,812	(23,036)	10,193
EBITDA									2,242,455	2,182,355	1,672,004
Real estate at cost	17,627,011				17,387,701				17,627,011	17,387,701	17,293,970	17,140,726
Investments in partially owned entities	1,740,459				1,375,006				1,740,459	1,375,006	1,209,285
Total assets	20,446,487				20,517,471				20,446,487	20,517,471	20,185,472
Toys R Us [Member]
Income from partially owned entities									48,540	71,624	92,300
Investments in partially owned entities	506,809				447,334				506,809	447,334
All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									71,770	22,438	(19,910)
Investments in partially owned entities	1,233,650				927,672				1,233,650	927,672
New York Office [Member]
Property rentals									783,438	773,996	757,372
Straight Line Rent Adjustments									25,720	34,197	36,832
Amortization of acquired below-market leases, net									31,547	36,164	39,474
Operating Leases, Income Statement, Lease Revenue, Total									840,705	844,357	833,678
Tenant expense reimbursements									140,038	137,412	136,368
Cleveland Medical Mart development project									0
BMS cleaning fees									95,452	88,664	75,549
Management and leasing fees									7,394	6,192	4,211
Lease termination fees									11,539	4,270	1,840
Other									22,189	22,283	18,868
Total Revenues									1,117,317	1,103,178	1,070,514
Operating									485,731	469,495	451,977
Depreciation and amortization									186,765	176,534	173,433
General and administrative									18,815	18,578	22,662
Cleveland Medical Mart development project									0
Tenant buy-outs, impairment losses and other acquisition related costs									0	0	0
Total expenses									691,311	664,607	648,072
Operating income									426,006	438,571	422,442
Income from partially owned entities									(12,559)	(6,354)	5,817
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									0	0
Interest and other investment (loss) income, net									642	608	876
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(138,336)	(132,279)	(133,647)
Net gain on early extinguishment of debt										0	0
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									0	0	0
Income before income taxes									275,753	300,546	295,488
Income tax expense									(2,084)	(2,167)	(1,332)
Income from continuing operations									273,669	298,379	294,156
Income (loss) from discontinued operations									563	168	945
Net income									274,232	298,547	295,101
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									(10,042)	(9,559)	(9,098)
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									0	0	0
Net income (loss) attributable to Common Shareholders									264,190	288,988	286,003
Interest and debt expense (2)									150,627	126,209	126,968
Depreciation and amortization (2)									201,122	170,505	168,517
Income tax (benefit) expense (2)									2,204	2,167	1,332
EBITDA									618,143	587,869	582,820
Real estate at cost	5,554,964				5,505,010				5,554,964	5,505,010	5,421,640
Investments in partially owned entities	355,499				97,743				355,499	97,743	128,961
Total assets	6,244,822				5,743,781				6,244,822	5,743,781	5,538,362
New York Office [Member] | Toys R Us [Member]
Income from partially owned entities									0	0	0
New York Office [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									(12,559)	(6,354)	5,817
Washington DC Office [Member]
Property rentals									558,256	566,041	526,683
Straight Line Rent Adjustments									(721)	5,849	22,683
Amortization of acquired below-market leases, net									2,088	2,326	3,452
Operating Leases, Income Statement, Lease Revenue, Total									559,623	574,216	552,818
Tenant expense reimbursements									36,849	51,963	60,620
Cleveland Medical Mart development project									0
BMS cleaning fees									0	0	0
Management and leasing fees									12,361	15,934	8,183
Lease termination fees									3,794	1,148	2,224
Other									20,650	21,427	47,745
Total Revenues									633,277	664,688	671,590
Operating									200,677	213,935	220,333
Depreciation and amortization									160,729	142,720	142,415
General and administrative									26,380	25,464	26,205
Cleveland Medical Mart development project									0
Tenant buy-outs, impairment losses and other acquisition related costs									0	0	24,875
Total expenses									387,786	382,119	413,828
Operating income									245,491	282,569	257,762
Income from partially owned entities									(6,381)	(564)	4,850
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									0	0
Interest and other investment (loss) income, net									199	157	786
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(120,724)	(130,540)	(128,039)
Net gain on early extinguishment of debt										0	0
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									0	54,742	0
Income before income taxes									118,585	206,364	135,359
Income tax expense									(2,927)	(1,816)	(1,482)
Income from continuing operations									115,658	204,548	133,877
Income (loss) from discontinued operations									46,466	(4,481)	52,308
Net income									162,124	200,067	186,185
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									0	0	0
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									0	0	0
Net income (loss) attributable to Common Shareholders									162,124	200,067	186,185
Interest and debt expense (2)									134,270	136,174	132,610
Depreciation and amortization (2)									181,560	159,283	152,747
Income tax (benefit) expense (2)									3,123	2,027	1,590
EBITDA									481,077	497,551	473,132
Real estate at cost	4,373,361				4,237,438				4,373,361	4,237,438	4,593,749
Investments in partially owned entities	113,536				149,295				113,536	149,295	119,182
Total assets	4,150,140				3,872,209				4,150,140	3,872,209	4,138,752
Washington DC Office [Member] | Toys R Us [Member]
Income from partially owned entities									0	0	0
Washington DC Office [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									(6,381)	(564)	4,850
Retail [Member]
Property rentals									424,646	390,068	354,397
Straight Line Rent Adjustments									16,319	28,604	26,943
Amortization of acquired below-market leases, net									23,751	21,470	22,095
Operating Leases, Income Statement, Lease Revenue, Total									464,716	440,142	403,435
Tenant expense reimbursements									150,338	144,224	132,385
Cleveland Medical Mart development project									0
BMS cleaning fees									0	0	0
Management and leasing fees									3,071	1,029	1,731
Lease termination fees									767	7,641	464
Other									5,966	3,674	2,565
Total Revenues									624,858	596,710	540,580
Operating									205,385	220,090	200,457
Depreciation and amortization									114,360	108,156	99,217
General and administrative									28,098	29,610	30,339
Cleveland Medical Mart development project									0
Tenant buy-outs, impairment losses and other acquisition related costs									24,146	72,500	9,589
Total expenses									371,989	430,356	339,602
Operating income									252,869	166,354	200,978
Income from partially owned entities									4,006	9,401	4,728
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									0	0
Interest and other investment (loss) income, net									(29)	180	69
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(91,895)	(85,063)	(88,844)
Net gain on early extinguishment of debt										105,571	769
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									4,278	0	0
Income before income taxes									169,229	196,443	117,700
Income tax expense									(34)	(37)	(319)
Income from continuing operations									169,195	196,406	117,381
Income (loss) from discontinued operations									4,000	2,453	(3,430)
Net income									173,195	198,859	113,951
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									237	(778)	915
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									0	0	0
Net income (loss) attributable to Common Shareholders									173,432	198,081	114,866
Interest and debt expense (2)									96,644	92,653	95,990
Depreciation and amortization (2)									117,716	114,335	105,903
Income tax (benefit) expense (2)									34	37	319
EBITDA									387,826	405,106	317,078
Real estate at cost	4,828,536				4,782,697				4,828,536	4,782,697	4,517,625
Investments in partially owned entities	13,264				11,831				13,264	11,831	22,955
Total assets	4,438,198				4,284,871				4,438,198	4,284,871	3,511,987
Retail [Member] | Toys R Us [Member]
Income from partially owned entities									0	0	0
Retail [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									4,006	9,401	4,728
Merchandise Mart [Member]
Property rentals									208,059	199,323	191,485
Straight Line Rent Adjustments									(2,680)	382	2,478
Amortization of acquired below-market leases, net									38	(75)	89
Operating Leases, Income Statement, Lease Revenue, Total									205,417	199,630	194,052
Tenant expense reimbursements									11,602	11,059	12,079
Cleveland Medical Mart development project									154,080
BMS cleaning fees									0	0	0
Management and leasing fees									342	156	88
Lease termination fees									295	467	219
Other									3,558	3,838	7,528
Total Revenues									375,294	215,150	213,966
Operating									132,470	114,161	113,078
Depreciation and amortization									41,094	40,130	41,587
General and administrative									29,996	26,720	30,749
Cleveland Medical Mart development project									145,824
Tenant buy-outs, impairment losses and other acquisition related costs									28,228	20,000	0
Total expenses									377,612	201,011	185,414
Operating income									(2,318)	14,139	28,552
Income from partially owned entities									455	(179)	151
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									0	0
Interest and other investment (loss) income, net									43	47	95
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(36,873)	(37,932)	(38,009)
Net gain on early extinguishment of debt										0	0
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									0	765	0
Income before income taxes									(38,693)	(23,160)	(9,211)
Income tax expense									(2,237)	(173)	(2,140)
Income from continuing operations									(40,930)	(23,333)	(11,351)
Income (loss) from discontinued operations									94,504	(5,284)	106
Net income									53,574	(28,617)	(11,245)
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									0	0	0
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									0	0	0
Net income (loss) attributable to Common Shareholders									53,574	(28,617)	(11,245)
Interest and debt expense (2)									40,916	61,379	52,862
Depreciation and amortization (2)									46,725	51,064	56,702
Income tax (benefit) expense (2)									2,237	232	2,208
EBITDA									143,452	84,058	100,527
Real estate at cost	963,811				970,417				963,811	970,417	992,290
Investments in partially owned entities	3,589				4,183				3,589	4,183	6,520
Total assets	1,226,084				1,435,714				1,226,084	1,435,714	1,455,000
Merchandise Mart [Member] | Toys R Us [Member]
Income from partially owned entities									0	0	0
Merchandise Mart [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									455	(179)	151
Toys R Us Segment [Member]
Property rentals									0	0	0
Straight Line Rent Adjustments									0	0	0
Amortization of acquired below-market leases, net									0	0	0
Operating Leases, Income Statement, Lease Revenue, Total									0	0	0
Tenant expense reimbursements									0	0	0
Cleveland Medical Mart development project									0
BMS cleaning fees									0	0	0
Management and leasing fees									0	0	0
Lease termination fees									0	0	0
Other									0	0	0
Total Revenues									0	0	0
Operating									0	0	0
Depreciation and amortization									0	0	0
General and administrative									0	0	0
Cleveland Medical Mart development project									0
Tenant buy-outs, impairment losses and other acquisition related costs									0	0	0
Total expenses									0	0	0
Operating income									0	0	0
Income from partially owned entities									48,540	71,624	92,300
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									0	0
Interest and other investment (loss) income, net									0	0	0
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									0	0	0
Net gain on early extinguishment of debt										0	0
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									0	0	0
Income before income taxes									48,540	71,624	92,300
Income tax expense									0	0	0
Income from continuing operations									48,540	71,624	92,300
Income (loss) from discontinued operations									0	0	0
Net income									48,540	71,624	92,300
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									0	0	0
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									0	0	0
Net income (loss) attributable to Common Shareholders									48,540	71,624	92,300
Interest and debt expense (2)									157,135	177,272	127,390
Depreciation and amortization (2)									134,967	131,284	132,227
Income tax (benefit) expense (2)									(1,132)	(45,418)	(13,185)
EBITDA									339,510	334,762	338,732
Real estate at cost	0				0				0	0	0
Investments in partially owned entities	506,809				447,334				506,809	447,334	409,453
Total assets	506,809				447,334				506,809	447,334	409,453
Toys R Us Segment [Member] | Toys R Us [Member]
Income from partially owned entities									48,540	71,624	92,300
Toys R Us Segment [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									0	0	0
Other Segment [Member]
Property rentals									183,539	169,730	159,232
Straight Line Rent Adjustments									2,793	3,975	469
Amortization of acquired below-market leases, net									5,018	5,657	5,291
Operating Leases, Income Statement, Lease Revenue, Total									191,350	179,362	164,992
Tenant expense reimbursements									10,593	10,958	9,838
Cleveland Medical Mart development project									0
BMS cleaning fees									(33,698)	(30,611)	(21,725)
Management and leasing fees									(3,065)	(3,194)	(2,757)
Lease termination fees									0	1,300	139
Other									(261)	3,140	8,454
Total Revenues									164,919	160,955	158,941
Operating									67,334	65,163	64,700
Depreciation and amortization									50,863	54,482	62,882
General and administrative									106,692	113,577	120,629
Cleveland Medical Mart development project									0
Tenant buy-outs, impairment losses and other acquisition related costs									5,925	36,958	39,299
Total expenses									230,814	270,180	287,510
Operating income									(65,895)	(109,225)	(128,569)
Income from partially owned entities									86,249	20,134	(35,456)
Income (loss) from Real Estate Fund (of which $13,598 and $(806), respectively, are attributable to noncontrolling interests)									22,886	(303)
Interest and other investment (loss) income, net									147,971	234,323	(118,176)
Interest and debt expense (including amortization of deferred financing costs of $20,729, $18,542 and $17,593 respectively)									(156,187)	(174,238)	(229,229)
Net gain on early extinguishment of debt										(10,782)	(26,684)
Net gain on disposition of wholly owned and partially owned assets other than depreciable real estate									10,856	25,925	5,641
Income before income taxes									45,880	(14,166)	(532,473)
Income tax expense									(17,545)	(18,283)	(15,369)
Income from continuing operations									28,335	(32,449)	(547,842)
Income (loss) from discontinued operations									0	0	0
Net income									28,335	(32,449)	(547,842)
Net (income) loss attributable to noncontrolling interests in consolidated subsidiaries									(11,981)	5,417	11,022
Net (income) attributable to noncontrolling interests in the Operating Partnership, including unit distributions									(55,912)	(55,228)	(25,120)
Net income (loss) attributable to Common Shareholders									(39,558)	(82,260)	(561,940)
Interest and debt expense (2)									218,328	234,395	291,007
Depreciation and amortization (2)									95,331	102,955	112,719
Income tax (benefit) expense (2)									(1,654)	17,919	17,929
EBITDA									272,447	273,009	(140,285)
Real estate at cost	1,906,339				1,892,139				1,906,339	1,892,139	1,768,666
Investments in partially owned entities	747,762				664,620				747,762	664,620	522,214
Total assets	3,880,434				4,733,562				3,880,434	4,733,562	5,131,918
Other Segment [Member] | Toys R Us [Member]
Income from partially owned entities									0	0	0
Other Segment [Member] | All Equity Method Investees Excluding Toys [Member]
Income from partially owned entities									$ 86,249	$ 20,134	$ (35,456)