Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 646,490	$ 597,209
Rental Revenue: Residential	4,049	6,854
Rental Revenue: Hotel	8,757	9,085
Total revenue	$ 659,296	$ 613,148
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Office	$ 217,572	$ 217,804
Rental Expenses: Residential	1,600	3,546
Rental Expenses: Hotel	7,634	7,576
Rental Expenses: Total	$ 226,806	$ 228,926
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net operating Income	$ 432,490	$ 384,222
Net operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 177,827	$ 176,027
Rental Revenue: Residential	1,171	1,178
Rental Revenue: Hotel	8,757	9,085
Total revenue	$ 187,755	$ 186,290
Rental Revenue: % of Grand Totals	28.48%	30.38%
Rental Expenses: Office	$ 70,687	$ 76,451
Rental Expenses: Residential	520	509
Rental Expenses: Hotel	7,634	7,576
Rental Expenses: Total	$ 78,841	$ 84,536
Rental Expenses: % Of Grand Totals	34.76%	36.92%
Net operating Income	$ 108,914	$ 101,754
Net operating Income: % of Grand Totals	25.18%	26.48%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 291,858	$ 253,098
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 291,858	$ 253,098
Rental Revenue: % of Grand Totals	44.27%	41.28%
Rental Expenses: Office	$ 88,798	$ 85,061
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 88,798	$ 85,061
Rental Expenses: % Of Grand Totals	39.15%	37.16%
Net operating Income	$ 203,060	$ 168,037
Net operating Income: % of Grand Totals	46.95%	43.73%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 76,317	$ 71,911
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 76,317	$ 71,911
Rental Revenue: % of Grand Totals	11.57%	11.73%
Rental Expenses: Office	$ 23,905	$ 22,821
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 23,905	$ 22,821
Rental Expenses: % Of Grand Totals	10.54%	9.97%
Net operating Income	$ 52,412	$ 49,090
Net operating Income: % of Grand Totals	12.12%	12.78%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 100,488	$ 96,173
Rental Revenue: Residential	2,878	5,676
Rental Revenue: Hotel	0	0
Total revenue	$ 103,366	$ 101,849
Rental Revenue: % of Grand Totals	15.68%	16.61%
Rental Expenses: Office	$ 34,182	$ 33,471
Rental Expenses: Residential	1,080	3,037
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 35,262	$ 36,508
Rental Expenses: % Of Grand Totals	15.55%	15.95%
Net operating Income	$ 68,104	$ 65,341
Net operating Income: % of Grand Totals	15.75%	17.01%