Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 539,906	$ 445,377
Rental Revenue: Office/Technical	15,697	9,847
Rental Revenue: Residential	5,682	5,578
Rental Revenue: Hotel	8,193	8,291
Total revenue	569,478	469,093
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Class A Office	198,521	163,503
Rental Expenses: Office/Technical	4,105	2,826
Rental Expenses: Residential	3,762	2,733
Rental Expenses: Hotel	6,797	7,044
Rental Expenses: Total	213,185	176,106
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net operating Income	356,293	292,987
Net operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	170,942	160,824
Rental Revenue: Office/Technical	5,820	5,653
Rental Revenue: Residential	1,163	1,067
Rental Revenue: Hotel	8,193	8,291
Total revenue	186,118	175,835
Rental Revenue: % of Grand Totals	32.68%	37.48%
Rental Expenses: Class A Office	71,398	65,287
Rental Expenses: Office/Technical	1,689	1,787
Rental Expenses: Residential	448	443
Rental Expenses: Hotel	6,797	7,044
Rental Expenses: Total	80,332	74,561
Rental Expenses: % Of Grand Totals	37.68%	42.35%
Net operating Income	105,786	101,274
Net operating Income: % of Grand Totals	29.69%	34.57%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	217,308	138,523
Rental Revenue: Office/Technical	0	0
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	217,308	138,523
Rental Revenue: % of Grand Totals	38.16%	29.53%
Rental Expenses: Class A Office	74,371	49,088
Rental Expenses: Office/Technical	0	0
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	74,371	49,088
Rental Expenses: % Of Grand Totals	34.89%	27.87%
Net operating Income	142,937	89,435
Net operating Income: % of Grand Totals	40.12%	30.53%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	54,608	52,671
Rental Revenue: Office/Technical	6,217	151
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	60,825	52,822
Rental Revenue: % of Grand Totals	10.68%	11.26%
Rental Expenses: Class A Office	19,313	18,373
Rental Expenses: Office/Technical	1,214	35
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	20,527	18,408
Rental Expenses: % Of Grand Totals	9.63%	10.45%
Net operating Income	40,298	34,414
Net operating Income: % of Grand Totals	11.31%	11.75%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	97,048	93,359
Rental Revenue: Office/Technical	3,660	4,043
Rental Revenue: Residential	4,519	4,511
Rental Revenue: Hotel	0	0
Total revenue	105,227	101,913
Rental Revenue: % of Grand Totals	18.48%	21.73%
Rental Expenses: Class A Office	33,439	30,755
Rental Expenses: Office/Technical	1,202	1,004
Rental Expenses: Residential	3,314	2,290
Rental Expenses: Hotel	0	0
Rental Expenses: Total	37,955	34,049
Rental Expenses: % Of Grand Totals	17.80%	19.33%
Net operating Income	$ 67,272	$ 67,864
Net operating Income: % of Grand Totals	18.88%	23.15%