Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 2,505,272	$ 2,460,953	$ 2,403,338
Rental Revenue: Residential	16,596	16,699	18,883
Rental Revenue: Hotel	45,603	44,884	46,046
Rental Revenue Total	$ 2,567,471	$ 2,522,536	$ 2,468,267
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 923,675	$ 882,692	$ 864,025
Rental Expenses: Residential	6,302	7,076	8,227
Rental Expenses: Hotel	32,059	31,466	32,084
Rental Expenses: Total	$ 962,036	$ 921,234	$ 904,336
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net operating Income	$ 1,605,435	$ 1,601,302	$ 1,563,931
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 776,279	$ 727,265	$ 716,246
Rental Revenue: Residential	4,745	4,812	4,801
Rental Revenue: Hotel	45,603	44,884	46,046
Rental Revenue Total	$ 826,627	$ 776,961	$ 767,093
Rental Revenue: % of Grand Totals	32.20%	30.80%	31.08%
Rental Expenses: Class A Office	$ 301,097	$ 282,827	$ 287,341
Rental Expenses: Residential	2,044	2,708	2,006
Rental Expenses: Hotel	32,059	31,466	32,084
Rental Expenses: Total	$ 335,200	$ 317,001	$ 321,431
Rental Expenses: % Of Grand Totals	34.84%	34.41%	35.54%
Net operating Income	$ 491,427	$ 459,960	$ 445,662
Net operating Income: % of Grand Totals	30.61%	28.73%	28.50%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 969,371	$ 1,012,518	$ 1,000,030
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 969,371	$ 1,012,518	$ 1,000,030
Rental Revenue: % of Grand Totals	37.75%	40.14%	40.52%
Rental Expenses: Class A Office	$ 372,810	$ 363,188	$ 346,897
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 372,810	$ 363,188	$ 346,897
Rental Expenses: % Of Grand Totals	38.76%	39.42%	38.36%
Net operating Income	$ 596,561	$ 649,330	$ 653,133
Net operating Income: % of Grand Totals	37.15%	40.55%	41.76%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 345,519	$ 318,609	$ 302,434
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 345,519	$ 318,609	$ 302,434
Rental Revenue: % of Grand Totals	13.46%	12.63%	12.25%
Rental Expenses: Class A Office	$ 105,253	$ 100,787	$ 98,206
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 105,253	$ 100,787	$ 98,206
Rental Expenses: % Of Grand Totals	10.94%	10.94%	10.86%
Net operating Income	$ 240,266	$ 217,822	$ 204,228
Net operating Income: % of Grand Totals	14.97%	13.60%	13.06%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 414,103	$ 402,561	$ 384,628
Rental Revenue: Residential	11,851	11,887	14,082
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 425,954	$ 414,448	$ 398,710
Rental Revenue: % of Grand Totals	16.59%	16.43%	16.15%
Rental Expenses: Class A Office	$ 144,515	$ 135,890	$ 131,581
Rental Expenses: Residential	4,258	4,368	6,221
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 148,773	$ 140,258	$ 137,802
Rental Expenses: % Of Grand Totals	15.46%	15.23%	15.24%
Net operating Income	$ 277,181	$ 274,190	$ 260,908
Net operating Income: % of Grand Totals	17.27%	17.12%	16.68%