Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 2,590,659	$ 2,505,272	$ 2,460,953
Rental Revenue: Residential	22,551	16,596	16,699
Rental Revenue: Hotel	49,118	45,603	44,884
Rental Revenue Total	$ 2,662,328	$ 2,567,471	$ 2,522,536
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 966,547	$ 923,675	$ 882,692
Rental Expenses: Residential	12,604	6,302	7,076
Rental Expenses: Hotel	33,863	32,059	31,466
Rental Expenses: Total	$ 1,013,014	$ 962,036	$ 921,234
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net Operating Income	$ 1,649,314	$ 1,605,435	$ 1,601,302
Net Operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 838,341	$ 776,279	$ 727,265
Rental Revenue: Residential	6,694	4,745	4,812
Rental Revenue: Hotel	49,118	45,603	44,884
Rental Revenue Total	$ 894,153	$ 826,627	$ 776,961
Rental Revenue: % of Grand Totals	33.59%	32.20%	30.80%
Rental Expenses: Class A Office	$ 315,653	$ 301,097	$ 282,827
Rental Expenses: Residential	3,632	2,044	2,708
Rental Expenses: Hotel	33,863	32,059	31,466
Rental Expenses: Total	$ 353,148	$ 335,200	$ 317,001
Rental Expenses: % Of Grand Totals	34.86%	34.84%	34.41%
Net Operating Income	$ 541,005	$ 491,427	$ 459,960
Net Operating Income: % of Grand Totals	32.80%	30.61%	28.73%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 959,050	$ 969,371	$ 1,012,518
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 959,050	$ 969,371	$ 1,012,518
Rental Revenue: % of Grand Totals	36.02%	37.75%	40.14%
Rental Expenses: Class A Office	$ 377,992	$ 372,810	$ 363,188
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 377,992	$ 372,810	$ 363,188
Rental Expenses: % Of Grand Totals	37.32%	38.76%	39.42%
Net Operating Income	$ 581,058	$ 596,561	$ 649,330
Net Operating Income: % of Grand Totals	35.23%	37.15%	40.55%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 397,180	$ 345,519	$ 318,609
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 397,180	$ 345,519	$ 318,609
Rental Revenue: % of Grand Totals	14.92%	13.46%	12.63%
Rental Expenses: Class A Office	$ 130,016	$ 105,253	$ 100,787
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 130,016	$ 105,253	$ 100,787
Rental Expenses: % Of Grand Totals	12.83%	10.94%	10.94%
Net Operating Income	$ 267,164	$ 240,266	$ 217,822
Net Operating Income: % of Grand Totals	16.20%	14.97%	13.60%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 396,088	$ 414,103	$ 402,561
Rental Revenue: Residential	15,857	11,851	11,887
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 411,945	$ 425,954	$ 414,448
Rental Revenue: % of Grand Totals	15.47%	16.59%	16.43%
Rental Expenses: Class A Office	$ 142,886	$ 144,515	$ 135,890
Rental Expenses: Residential	8,972	4,258	4,368
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 151,858	$ 148,773	$ 140,258
Rental Expenses: % Of Grand Totals	14.99%	15.46%	15.23%
Net Operating Income	$ 260,087	$ 277,181	$ 274,190
Net Operating Income: % of Grand Totals	15.77%	17.27%	17.12%