Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 631,957	$ 601,117	$ 1,246,337	$ 1,247,607
Rental Revenue: Residential	4,210	4,088	8,166	8,137
Rental Revenue: Hotel	13,375	12,808	20,795	21,565
Total revenue	$ 649,542	$ 618,013	$ 1,275,298	$ 1,277,309
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Office	$ 228,819	$ 216,332	$ 455,555	$ 433,904
Rental Expenses: Residential	1,635	1,606	3,186	3,206
Rental Expenses: Hotel	8,404	7,978	15,495	15,612
Rental Expenses: Total	$ 238,858	$ 225,916	$ 474,236	$ 452,722
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	$ 410,684	$ 392,097	$ 801,062	$ 824,587
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 191,760	$ 179,048	$ 377,196	$ 356,875
Rental Revenue: Residential	1,153	1,180	2,292	2,351
Rental Revenue: Hotel	13,375	12,808	20,795	21,565
Total revenue	$ 206,288	$ 193,036	$ 400,283	$ 380,791
Rental Revenue: % of Grand Totals	31.76%	31.24%	31.39%	29.81%
Rental Expenses: Office	$ 74,160	$ 68,754	$ 149,416	$ 139,441
Rental Expenses: Residential	545	513	1,040	1,033
Rental Expenses: Hotel	8,404	7,978	15,495	15,612
Rental Expenses: Total	$ 83,109	$ 77,245	$ 165,951	$ 156,086
Rental Expenses: % Of Grand Totals	34.79%	34.19%	34.99%	34.47%
Net operating Income	$ 123,179	$ 115,791	$ 234,332	$ 224,705
Net operating Income: % of Grand Totals	29.99%	29.53%	29.25%	27.25%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 251,844	$ 243,957	$ 493,414	$ 535,815
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 251,844	$ 243,957	$ 493,414	$ 535,815
Rental Revenue: % of Grand Totals	38.77%	39.47%	38.69%	41.95%
Rental Expenses: Office	$ 93,110	$ 88,749	$ 184,794	$ 177,547
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 93,110	$ 88,749	$ 184,794	$ 177,547
Rental Expenses: % Of Grand Totals	38.98%	39.29%	38.97%	39.22%
Net operating Income	$ 158,734	$ 155,208	$ 308,620	$ 358,268
Net operating Income: % of Grand Totals	38.65%	39.59%	38.53%	43.45%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 85,483	$ 78,524	$ 170,124	$ 154,841
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 85,483	$ 78,524	$ 170,124	$ 154,841
Rental Revenue: % of Grand Totals	13.16%	12.71%	13.34%	12.12%
Rental Expenses: Office	$ 25,938	$ 25,470	$ 50,412	$ 49,375
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 25,938	$ 25,470	$ 50,412	$ 49,375
Rental Expenses: % Of Grand Totals	10.86%	11.27%	10.63%	10.91%
Net operating Income	$ 59,545	$ 53,054	$ 119,712	$ 105,466
Net operating Income: % of Grand Totals	14.50%	13.53%	14.94%	12.79%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 102,870	$ 99,588	$ 205,603	$ 200,076
Rental Revenue: Residential	3,057	2,908	5,874	5,786
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 105,927	$ 102,496	$ 211,477	$ 205,862
Rental Revenue: % of Grand Totals	16.31%	16.58%	16.58%	16.12%
Rental Expenses: Office	$ 35,611	$ 33,359	$ 70,933	$ 67,541
Rental Expenses: Residential	1,090	1,093	2,146	2,173
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 36,701	$ 34,452	$ 73,079	$ 69,714
Rental Expenses: % Of Grand Totals	15.37%	15.25%	15.41%	15.40%
Net operating Income	$ 69,226	$ 68,044	$ 138,398	$ 136,148
Net operating Income: % of Grand Totals	16.86%	17.35%	17.28%	16.51%