Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 692,544		$ 634,663		$ 1,386,124	$ 1,359,322
Rental Revenue: Residential	[1]	9,763		9,402		18,938	19,358
Rental Revenue Total	[1]	$ 703,868		$ 644,164		$ 1,407,255	$ 1,385,604
Rental Revenue: % of Grand Totals		100.00%	[1]	100.00%	[1]	100.00%	100.00%
Rental Expenses: Class A Office		$ 242,775		$ 235,822		$ 494,037	$ 494,724
Rental Expenses: Residential		5,928		3,965		12,055	8,029
Rental Expenses: Total		$ 250,699		$ 241,760		$ 510,139	$ 511,547
Rental Expenses: % Of Grand Totals		100.00%		100.00%		100.00%	100.00%
Net Operating Income		$ 453,169		$ 402,404		$ 897,116	$ 874,057
Net Operating Income: % of Grand Totals		100.00%		100.00%		100.00%	100.00%
Company's Share of Net Operating Income: % of Grand Totals		100.00%		100.00%		100.00%	100.00%
Total revenue		$ 713,807		$ 654,773		$ 1,427,502	$ 1,407,329
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	229,571		218,351		459,974	457,849
Rental Revenue: Residential	[1]	3,131		3,401		6,176	7,469
Rental Revenue Total	[1]	$ 234,263		$ 221,851		$ 468,343	$ 472,242
Rental Revenue: % of Grand Totals		33.28%	[1]	34.43%	[1]	33.28%	34.09%
Rental Expenses: Class A Office		$ 78,165		$ 75,694		$ 158,046	$ 158,239
Rental Expenses: Residential		1,435		1,235		2,890	2,575
Rental Expenses: Total		$ 81,596		$ 78,902		$ 164,983	$ 169,608
Rental Expenses: % Of Grand Totals		32.55%		32.64%		32.34%	33.16%
Net Operating Income		$ 152,667		$ 142,949		$ 303,360	$ 302,634
Net Operating Income: % of Grand Totals		33.69%		35.52%		33.82%	34.63%
Company's Share of Net Operating Income: % of Grand Totals		33.70%		33.92%		33.68%	33.76%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 0		$ 0		$ 0	$ 0
Rental Revenue: Residential	[1]	0		0		0	0
Rental Revenue Total	[1]	$ 0		$ 0		$ 0	$ 0
Rental Revenue: % of Grand Totals		0.00%	[1]	0.00%	[1]	0.00%	0.00%
Rental Expenses: Class A Office		$ 0		$ 0		$ 0	$ 0
Rental Expenses: Residential		0		0		0	0
Rental Expenses: Total		$ 0		$ 0		$ 0	$ 0
Rental Expenses: % Of Grand Totals		0.00%		0.00%		0.00%	0.00%
Net Operating Income		$ 0		$ 0		$ 0	$ 0
Net Operating Income: % of Grand Totals		0.00%		0.00%		0.00%	0.00%
Company's Share of Net Operating Income: % of Grand Totals		2.84%		3.78%		3.09%	3.64%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 252,182		$ 204,242		$ 502,346	$ 459,528
Rental Revenue: Residential	[1]	0		0		0	0
Rental Revenue Total	[1]	$ 252,182		$ 204,242		$ 502,346	$ 459,528
Rental Revenue: % of Grand Totals		35.84%	[1]	31.71%	[1]	35.70%	33.16%
Rental Expenses: Class A Office		$ 92,662		$ 88,367		$ 192,047	$ 187,507
Rental Expenses: Residential		0		0		0	0
Rental Expenses: Total		$ 92,662		$ 88,367		$ 192,047	$ 187,507
Rental Expenses: % Of Grand Totals		36.96%		36.54%		37.65%	36.65%
Net Operating Income		$ 159,520		$ 115,875		$ 310,299	$ 272,021
Net Operating Income: % of Grand Totals		35.20%		28.80%		34.59%	31.12%
Company's Share of Net Operating Income: % of Grand Totals		28.68%		23.84%		27.99%	25.24%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 126,181		$ 128,233		$ 256,779	$ 264,972
Rental Revenue: Residential	[1]	690		0		1,011	0
Rental Revenue Total	[1]	$ 126,871		$ 128,233		$ 257,790	$ 264,972
Rental Revenue: % of Grand Totals		18.02%	[1]	19.91%	[1]	18.32%	19.12%
Rental Expenses: Class A Office		$ 40,954		$ 39,081		$ 81,203	$ 81,650
Rental Expenses: Residential		1,544		0		3,230	0
Rental Expenses: Total		$ 42,498		$ 39,081		$ 84,433	$ 81,650
Rental Expenses: % Of Grand Totals		16.95%		16.17%		16.55%	15.96%
Net Operating Income		$ 84,373		$ 89,152		$ 173,357	$ 183,322
Net Operating Income: % of Grand Totals		18.62%		22.15%		19.32%	20.97%
Company's Share of Net Operating Income: % of Grand Totals		20.35%		23.44%		21.06%	22.41%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 84,610		$ 83,837		$ 167,025	$ 176,973
Rental Revenue: Residential	[1]	5,942		6,001		11,751	11,889
Rental Revenue Total	[1]	$ 90,552		$ 89,838		$ 178,776	$ 188,862
Rental Revenue: % of Grand Totals		12.86%	[1]	13.95%	[1]	12.70%	13.63%
Rental Expenses: Class A Office		$ 30,994		$ 32,680		$ 62,741	$ 67,328
Rental Expenses: Residential		2,949		2,730		5,935	5,454
Rental Expenses: Total		$ 33,943		$ 35,410		$ 68,676	$ 72,782
Rental Expenses: % Of Grand Totals		13.54%		14.65%		13.46%	14.23%
Net Operating Income		$ 56,609		$ 54,428		$ 110,100	$ 116,080
Net Operating Income: % of Grand Totals		12.49%		13.53%		12.27%	13.28%
Company's Share of Net Operating Income: % of Grand Totals		14.43%		15.02%		14.18%	14.95%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	$ 1,561		$ 99		$ 2,193	$ 6,924
Operating expense		1,996		1,973		4,047	8,794
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	1,561		99		2,193	6,924
Operating expense		1,996		1,973		4,047	8,794
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0		0		0	0
Operating expense		0		0		0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0		0		0	0
Operating expense		0		0		0	0
Hotel [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0		0		0	0
Operating expense		0		0		0	0
Hotel [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0		0		0	0
Operating expense		0		0		0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(46,287)		(32,427)		(90,663)	(80,088)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(10,576)		(10,576)		(20,800)	(21,239)
Noncontrolling interest - property partnerships [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0		0		0	0
Noncontrolling interest - property partnerships [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(35,711)		(21,851)		(69,863)	(58,849)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0		0		0	0
Noncontrolling interest - property partnerships [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0		0		0	0
Unconsolidated Joint Ventures [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	91,226		89,563		178,492	182,766
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		3,624		2,627		5,905	5,726
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		12,265		15,026		26,457	30,956
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		172		815		(621)	1,571
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		3,580		4,127		7,060	7,286
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		5,776		5,316		11,411	11,130
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income		432,299		397,888		856,665	850,638
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		145,715		135,000		288,465	287,121
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		12,265		15,026		26,457	30,956
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		123,981		94,839		239,815	214,743
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		87,953		93,279		180,417	190,608
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		$ 62,385		$ 59,744		$ 121,511	$ 127,210

[1] Rental Revenue is equal to Total Revenue per the Company’s Consolidated Statements of Operations, less Development and Management Services Revenue and Direct Reimbursements of Payroll and Related Costs from Management Services Contracts Revenue per the Consolidated Statements of Operations.

[2] Includes straight-line rent adjustments of approximately $5.0 million and $8.5 million for the three months ended June 30, 2021 and 2020, respectively, and approximately $6.1 million and $18.2 million for the six months ended June 30, 2021 and 2020, respectively.