Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 601,117	$ 596,145	$ 1,247,607	$ 1,193,354
Rental Revenue: Residential	4,088	3,811	8,137	10,665
Rental Revenue: Hotel	12,808	13,403	21,565	22,488
Total revenue	$ 618,013	$ 613,359	$ 1,277,309	$ 1,226,507
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Office	$ 216,332	$ 212,933	$ 433,904	$ 430,737
Rental Expenses: Residential	1,606	1,531	3,206	5,077
Rental Expenses: Hotel	7,978	8,495	15,612	16,071
Rental Expenses: Total	$ 225,916	$ 222,959	$ 452,722	$ 451,885
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	$ 392,097	$ 390,400	$ 824,587	$ 774,622
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 179,048	$ 180,936	$ 356,875	$ 356,963
Rental Revenue: Residential	1,180	1,131	2,351	2,309
Rental Revenue: Hotel	12,808	13,403	21,565	22,488
Total revenue	$ 193,036	$ 195,470	$ 380,791	$ 381,760
Rental Revenue: % of Grand Totals	31.24%	31.87%	29.81%	31.13%
Rental Expenses: Office	$ 68,754	$ 69,653	$ 139,441	$ 146,104
Rental Expenses: Residential	513	511	1,033	1,020
Rental Expenses: Hotel	7,978	8,495	15,612	16,071
Rental Expenses: Total	$ 77,245	$ 78,659	$ 156,086	$ 163,195
Rental Expenses: % Of Grand Totals	34.19%	35.28%	34.47%	36.12%
Net operating Income	$ 115,791	$ 116,811	$ 224,705	$ 218,565
Net operating Income: % of Grand Totals	29.53%	29.92%	27.25%	28.22%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 243,957	$ 246,432	$ 535,815	$ 499,530
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 243,957	$ 246,432	$ 535,815	$ 499,530
Rental Revenue: % of Grand Totals	39.47%	40.18%	41.95%	40.73%
Rental Expenses: Office	$ 88,749	$ 85,661	$ 177,547	$ 170,722
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 88,749	$ 85,661	$ 177,547	$ 170,722
Rental Expenses: % Of Grand Totals	39.29%	38.42%	39.22%	37.78%
Net operating Income	$ 155,208	$ 160,771	$ 358,268	$ 328,808
Net operating Income: % of Grand Totals	39.59%	41.18%	43.45%	42.45%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 78,524	$ 74,335	$ 154,841	$ 146,246
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 78,524	$ 74,335	$ 154,841	$ 146,246
Rental Revenue: % of Grand Totals	12.71%	12.12%	12.12%	11.92%
Rental Expenses: Office	$ 25,470	$ 24,415	$ 49,375	$ 47,236
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 25,470	$ 24,415	$ 49,375	$ 47,236
Rental Expenses: % Of Grand Totals	11.27%	10.95%	10.91%	10.45%
Net operating Income	$ 53,054	$ 49,920	$ 105,466	$ 99,010
Net operating Income: % of Grand Totals	13.53%	12.79%	12.79%	12.78%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 99,588	$ 94,442	$ 200,076	$ 190,615
Rental Revenue: Residential	2,908	2,680	5,786	8,356
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 102,496	$ 97,122	$ 205,862	$ 198,971
Rental Revenue: % of Grand Totals	16.58%	15.83%	16.12%	16.22%
Rental Expenses: Office	$ 33,359	$ 33,204	$ 67,541	$ 66,675
Rental Expenses: Residential	1,093	1,020	2,173	4,057
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 34,452	$ 34,224	$ 69,714	$ 70,732
Rental Expenses: % Of Grand Totals	15.25%	15.35%	15.40%	15.65%
Net operating Income	$ 68,044	$ 62,898	$ 136,148	$ 128,239
Net operating Income: % of Grand Totals	17.35%	16.11%	16.51%	16.55%