Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 1,744,489	$ 1,635,164	$ 1,427,947
Rental Revenue: Office/Technical	39,218	41,585	41,348
Rental Revenue: Residential	20,568	6,617	0
Rental Revenue: Hotel	37,915	34,529	32,800
Rental Revenue: Total	1,842,190	1,717,895	1,502,095
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	635,757	573,220	487,098
Rental Expenses: Office/Technical	10,614	11,525	11,056
Rental Expenses: Residential	10,992	5,479	0
Rental Expenses: Hotel	28,120	26,128	25,153
Rental Expenses: Total	685,483	616,352	523,307
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net operating Income	1,156,707	1,101,543	978,788
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	620,586	540,924	366,200
Rental Revenue: Office/Technical	22,460	25,349	25,499
Rental Revenue: Residential	3,936	985	0
Rental Revenue: Hotel	37,915	34,529	32,800
Rental Revenue: Total	684,897	601,787	424,499
Rental Revenue: % of Grand Totals	37.18%	35.03%	28.26%
Rental Expenses: Class A Office	244,753	208,133	137,361
Rental Expenses: Office/Technical	6,499	7,245	6,888
Rental Expenses: Residential	1,675	521	0
Rental Expenses: Hotel	28,120	26,128	25,153
Rental Expenses: Total	281,047	242,027	169,402
Rental Expenses: % Of Grand Totals	41.00%	39.27%	32.37%
Net operating Income	403,850	359,760	255,097
Net operating Income: % of Grand Totals	34.91%	32.66%	26.06%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	481,844	458,791	445,296
Rental Revenue: Office/Technical	0	0	0
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue: Total	481,844	458,791	445,296
Rental Revenue: % of Grand Totals	26.16%	26.71%	29.65%
Rental Expenses: Class A Office	160,386	152,649	146,381
Rental Expenses: Office/Technical	0	0	0
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	160,386	152,649	146,381
Rental Expenses: % Of Grand Totals	23.40%	24.77%	27.97%
Net operating Income	321,458	306,142	298,915
Net operating Income: % of Grand Totals	27.79%	27.79%	30.54%
Princeton [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	61,350	62,648	65,475
Rental Revenue: Office/Technical	0	0	0
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue: Total	61,350	62,648	65,475
Rental Revenue: % of Grand Totals	3.33%	3.65%	4.36%
Rental Expenses: Class A Office	29,218	30,150	31,486
Rental Expenses: Office/Technical	0	0	0
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	29,218	30,150	31,486
Rental Expenses: % Of Grand Totals	4.26%	4.89%	6.02%
Net operating Income	32,132	32,498	33,989
Net operating Income: % of Grand Totals	2.78%	2.95%	3.47%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	214,316	213,257	215,468
Rental Revenue: Office/Technical	494	0	0
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue: Total	214,810	213,257	215,468
Rental Revenue: % of Grand Totals	11.66%	12.41%	14.34%
Rental Expenses: Class A Office	84,262	80,729	78,978
Rental Expenses: Office/Technical	149	0	0
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	84,411	80,729	78,978
Rental Expenses: % Of Grand Totals	12.31%	13.10%	15.09%
Net operating Income	130,399	132,528	136,490
Net operating Income: % of Grand Totals	11.27%	12.03%	13.95%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	366,393	359,544	335,508
Rental Revenue: Office/Technical	16,264	16,236	15,849
Rental Revenue: Residential	16,632	5,632	0
Rental Revenue: Hotel	0	0	0
Rental Revenue: Total	399,289	381,412	351,357
Rental Revenue: % of Grand Totals	21.67%	22.20%	23.39%
Rental Expenses: Class A Office	117,138	101,559	92,892
Rental Expenses: Office/Technical	3,966	4,280	4,168
Rental Expenses: Residential	9,317	4,958	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	130,421	110,797	97,060
Rental Expenses: % Of Grand Totals	19.03%	17.97%	18.55%
Net operating Income	$ 268,868	$ 270,615	$ 254,297
Net operating Income: % of Grand Totals	23.25%	24.57%	25.98%