Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Interest expense	$ 122,173	$ 105,030	$ 325,746	$ 308,168
General and administrative expense	24,841	21,617	94,673	72,208
Development and management services	5,479	8,024	22,072	25,733
Rental Revenue: Class A Office	537,080	434,015	1,458,025	1,280,007
Rental Revenue: Office/Technical	15,378	9,786	40,121	29,471
Rental Revenue: Residential	5,493	5,496	16,555	15,013
Rental Revenue: Hotel	10,652	9,359	30,061	26,224
Rental Revenue Total	568,603	458,656	1,544,762	1,350,715
Total revenue	574,082	466,680	1,566,834	1,376,448
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	193,329	160,088	528,031	461,130
Rental Expenses: Office/Technical	4,114	2,751	10,711	7,976
Rental Expenses: Residential	2,907	2,854	8,470	8,143
Rental Expenses: Hotel	6,580	6,886	20,959	19,601
Rental Expenses: Total	206,930	172,579	568,171	496,850
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net Operating Income	361,673	286,077	976,591	853,865
Net Operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Income from unconsolidated joint ventures	14,736	9,217	72,240	42,129
Interest and other income	3,879	4,001	6,646	8,029
Gains (Losses) from investments in securities	956	587	1,872	1,202
Gains from early extinguishments of debt	(30)	(5,494)	122	(4,453)
Transaction costs expensed	766	1,140	1,744	3,252
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	171,228	161,479	497,584	460,127
Rental Revenue: Office/Technical	5,662	5,539	16,853	16,882
Rental Revenue: Residential	1,125	995	3,257	2,891
Rental Revenue: Hotel	10,652	9,359	30,061	26,224
Rental Revenue Total	188,667	177,372	547,755	506,124
Rental Revenue: % of Grand Totals	33.18%	38.67%	35.46%	37.47%
Rental Expenses: Class A Office	67,798	65,261	196,710	183,601
Rental Expenses: Office/Technical	1,763	1,758	5,177	4,910
Rental Expenses: Residential	466	490	1,316	1,247
Rental Expenses: Hotel	6,580	6,886	20,959	19,601
Rental Expenses: Total	76,607	74,395	224,162	209,359
Rental Expenses: % Of Grand Totals	37.02%	43.11%	39.45%	42.14%
Net Operating Income	112,060	102,977	323,593	296,765
Net Operating Income: % of Grand Totals	30.98%	36.00%	33.14%	34.76%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	198,450	120,270	463,851	360,515
Rental Revenue: Office/Technical	0	0	0	0
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Rental Revenue Total	198,450	120,270	463,851	360,515
Rental Revenue: % of Grand Totals	34.90%	26.22%	30.02%	26.69%
Rental Expenses: Class A Office	66,322	41,088	156,414	118,531
Rental Expenses: Office/Technical	0	0	0	0
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	66,322	41,088	156,414	118,531
Rental Expenses: % Of Grand Totals	32.05%	23.81%	27.53%	23.86%
Net Operating Income	132,128	79,182	307,437	241,984
Net Operating Income: % of Grand Totals	36.53%	27.68%	31.48%	28.34%
Princeton [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	15,636	15,234	46,916	45,701
Rental Revenue: Office/Technical	0	0	0	0
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Rental Revenue Total	15,636	15,234	46,916	45,701
Rental Revenue: % of Grand Totals	2.75%	3.32%	3.04%	3.38%
Rental Expenses: Class A Office	7,033	6,790	21,386	20,889
Rental Expenses: Office/Technical	0	0	0	0
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	7,033	6,790	21,386	20,889
Rental Expenses: % Of Grand Totals	3.40%	3.93%	3.77%	4.20%
Net Operating Income	8,603	8,444	25,530	24,812
Net Operating Income: % of Grand Totals	2.38%	2.95%	2.61%	2.91%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	53,841	52,585	159,843	155,353
Rental Revenue: Office/Technical	5,839	148	11,317	344
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Rental Revenue Total	59,680	52,733	171,160	155,697
Rental Revenue: % of Grand Totals	10.50%	11.50%	11.08%	11.53%
Rental Expenses: Class A Office	20,006	19,530	58,036	56,496
Rental Expenses: Office/Technical	1,287	52	2,439	114
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	21,293	19,582	60,475	56,610
Rental Expenses: % Of Grand Totals	10.29%	11.35%	10.64%	11.39%
Net Operating Income	38,387	33,151	110,685	99,087
Net Operating Income: % of Grand Totals	10.62%	11.59%	11.33%	11.60%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	97,925	84,447	289,831	258,311
Rental Revenue: Office/Technical	3,877	4,099	11,951	12,245
Rental Revenue: Residential	4,368	4,501	13,298	12,122
Rental Revenue: Hotel	0	0	0	0
Rental Revenue Total	106,170	93,047	315,080	282,678
Rental Revenue: % of Grand Totals	18.67%	20.29%	20.40%	20.93%
Rental Expenses: Class A Office	32,170	27,419	95,485	81,613
Rental Expenses: Office/Technical	1,064	941	3,095	2,952
Rental Expenses: Residential	2,441	2,364	7,154	6,896
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	35,675	30,724	105,734	91,461
Rental Expenses: % Of Grand Totals	17.24%	17.80%	18.61%	18.41%
Net Operating Income	70,495	62,323	209,346	191,217
Net Operating Income: % of Grand Totals	19.49%	21.78%	21.44%	22.39%
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Interest expense	122,173	105,030	325,746	308,168
General and administrative expense	24,841	21,617	94,673	72,208
Development and management services	5,479	8,024	22,072	25,733
Income from unconsolidated joint ventures	14,736	9,217	72,240	42,129
Interest and other income	3,879	4,001	6,646	8,029
Gains (Losses) from investments in securities	956	587	1,872	1,202
Gains from early extinguishments of debt	(30)	(5,494)	122	(4,453)
Transaction costs expensed	$ 766	$ 1,140	$ 1,744	$ 3,252