Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 581,596	$ 548,699	$ 1,163,984	$ 1,088,605
Rental Revenue: Office/Technical	14,549	15,924	29,370	31,621
Rental Revenue: Residential	3,811	6,298	10,665	11,980
Rental Revenue: Hotel	13,403	12,367	22,488	20,560
Total revenue	$ 613,359	$ 583,288	$ 1,226,507	$ 1,152,766
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 209,131	$ 194,543	$ 422,846	$ 393,064
Rental Expenses: Office/Technical	3,802	4,167	7,891	8,272
Rental Expenses: Residential	1,531	3,936	5,077	7,698
Rental Expenses: Hotel	8,495	7,315	16,071	14,112
Rental Expenses: Total	$ 222,959	$ 209,961	$ 451,885	$ 423,146
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	$ 390,400	$ 373,327	$ 774,622	$ 729,620
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 174,894	$ 173,128	$ 344,801	$ 344,070
Rental Revenue: Office/Technical	6,042	5,912	12,162	11,732
Rental Revenue: Residential	1,131	1,089	2,309	2,252
Rental Revenue: Hotel	13,403	12,367	22,488	20,560
Total revenue	$ 195,470	$ 192,496	$ 381,760	$ 378,614
Rental Revenue: % of Grand Totals	31.87%	33.00%	31.13%	32.84%
Rental Expenses: Class A Office	$ 67,874	$ 65,818	$ 142,339	$ 137,216
Rental Expenses: Office/Technical	1,779	1,736	3,765	3,425
Rental Expenses: Residential	511	544	1,020	992
Rental Expenses: Hotel	8,495	7,315	16,071	14,112
Rental Expenses: Total	$ 78,659	$ 75,413	$ 163,195	$ 155,745
Rental Expenses: % Of Grand Totals	35.28%	35.92%	36.12%	36.81%
Net operating Income	$ 116,811	$ 117,083	$ 218,565	$ 222,869
Net operating Income: % of Grand Totals	29.92%	31.36%	28.22%	30.55%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 246,432	$ 223,899	$ 499,530	$ 441,207
Rental Revenue: Office/Technical	0	0	0	0
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 246,432	$ 223,899	$ 499,530	$ 441,207
Rental Revenue: % of Grand Totals	40.18%	38.39%	40.73%	38.28%
Rental Expenses: Class A Office	$ 85,661	$ 74,962	$ 170,722	$ 149,333
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	$ 85,661	$ 74,962	$ 170,722	$ 149,333
Rental Expenses: % Of Grand Totals	38.42%	35.70%	37.78%	35.29%
Net operating Income	$ 160,771	$ 148,937	$ 328,808	$ 291,874
Net operating Income: % of Grand Totals	41.18%	39.89%	42.45%	40.00%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 68,781	$ 57,450	$ 135,033	$ 112,058
Rental Revenue: Office/Technical	5,554	6,341	11,213	12,558
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 74,335	$ 63,791	$ 146,246	$ 124,616
Rental Revenue: % of Grand Totals	12.12%	10.94%	11.92%	10.81%
Rental Expenses: Class A Office	$ 23,416	$ 21,282	$ 45,325	$ 40,595
Rental Expenses: Office/Technical	999	1,308	1,911	2,522
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 24,415	$ 22,590	$ 47,236	$ 43,117
Rental Expenses: % Of Grand Totals	10.95%	10.76%	10.45%	10.19%
Net operating Income	$ 49,920	$ 41,201	$ 99,010	$ 81,499
Net operating Income: % of Grand Totals	12.79%	11.04%	12.78%	11.17%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 91,489	$ 94,222	$ 184,620	$ 191,270
Rental Revenue: Office/Technical	2,953	3,671	5,995	7,331
Rental Revenue: Residential	2,680	5,209	8,356	9,728
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 97,122	$ 103,102	$ 198,971	$ 208,329
Rental Revenue: % of Grand Totals	15.83%	17.67%	16.22%	18.07%
Rental Expenses: Class A Office	$ 32,180	$ 32,481	$ 64,460	$ 65,920
Rental Expenses: Office/Technical	1,024	1,123	2,215	2,325
Rental Expenses: Residential	1,020	3,392	4,057	6,706
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 34,224	$ 36,996	$ 70,732	$ 74,951
Rental Expenses: % Of Grand Totals	15.35%	17.62%	15.65%	17.71%
Net operating Income	$ 62,898	$ 66,106	$ 128,239	$ 133,378
Net operating Income: % of Grand Totals	16.11%	17.71%	16.55%	18.28%