Segment Reporting (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 415,905	$ 392,288
Rental Revenue: Office/Technical	12,311	11,550
Rental Revenue: Residential	4,481
Rental Revenue: Hotel	6,816	5,948
Rental Revenue: Total	439,513	409,786
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Class A Office	151,210	136,029
Rental Expenses: Office/Technical	3,581	3,601
Rental Expenses: Residential	2,715
Rental Expenses: Hotel	6,099	5,739
Rental Expenses: Total	163,605	145,369
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net Operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	140,871	127,429
Rental Revenue: Office/Technical	8,167	7,558
Rental Revenue: Residential	883
Rental Revenue: Hotel	6,816	5,948
Rental Revenue: Total	156,737	140,935
Rental Revenue: % of Grand Totals	35.66%	34.39%
Rental Expenses: Class A Office	57,890	47,901
Rental Expenses: Office/Technical	2,510	2,479
Rental Expenses: Residential	390
Rental Expenses: Hotel	6,099	5,739
Rental Expenses: Total	66,889	56,119
Rental Expenses: % Of Grand Totals	40.89%	38.61%
Net Operating Income	89,848	84,816
Net Operating Income: % of Grand Totals	32.56%	32.08%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	117,589	113,007
Rental Revenue: Residential
Rental Revenue: Total	117,589	113,007
Rental Revenue: % of Grand Totals	26.75%	27.58%
Rental Expenses: Class A Office	38,429	37,715
Rental Expenses: Residential
Rental Expenses: Total	38,429	37,715
Rental Expenses: % Of Grand Totals	23.49%	25.94%
Net Operating Income	79,160	75,292
Net Operating Income: % of Grand Totals	28.69%	28.48%
Princeton [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	14,850	16,279
Rental Revenue: Residential
Rental Revenue: Total	14,850	16,279
Rental Revenue: % of Grand Totals	3.38%	3.97%
Rental Expenses: Class A Office	7,358	7,915
Rental Expenses: Residential
Rental Expenses: Total	7,358	7,915
Rental Expenses: % Of Grand Totals	4.50%	5.44%
Net Operating Income	7,492	8,364
Net Operating Income: % of Grand Totals	2.72%	3.16%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	52,197	52,893
Rental Revenue: Office/Technical	52
Rental Revenue: Residential
Rental Revenue: Total	52,249	52,893
Rental Revenue: % of Grand Totals	11.89%	12.91%
Rental Expenses: Class A Office	20,091	19,333
Rental Expenses: Office/Technical	24
Rental Expenses: Residential
Rental Expenses: Total	20,115	19,333
Rental Expenses: % Of Grand Totals	12.29%	13.30%
Net Operating Income	32,134	33,560
Net Operating Income: % of Grand Totals	11.65%	12.69%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	90,398	82,680
Rental Revenue: Office/Technical	4,092	3,992
Rental Revenue: Residential	3,598
Rental Revenue: Total	98,088	86,672
Rental Revenue: % of Grand Totals	22.32%	21.15%
Rental Expenses: Class A Office	27,442	23,165
Rental Expenses: Office/Technical	1,047	1,122
Rental Expenses: Residential	2,325
Rental Expenses: Total	30,814	24,287
Rental Expenses: % Of Grand Totals	18.83%	16.71%
Net Operating Income	67,274	62,385
Net Operating Income: % of Grand Totals	24.38%	23.59%
Segment, Continuing Operations [Member]
Segment Reporting Information [Line Items]
Net Operating Income	$ 275,908	$ 264,417