Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 2,968,412	$ 2,792,160	$ 2,678,795
Rental Revenue: Residential	[1]	57,181	42,668	38,146
Rental Revenue Total	[1]	$ 3,065,075	$ 2,848,437	$ 2,724,419
Rental Revenue: % of Grand Totals		100.00%	100.00%	100.00%
Rental Expenses: Class A Office		$ 1,078,487	$ 996,707	$ 998,469
Rental Expenses: Residential		29,583	24,444	18,739
Rental Expenses: Total		$ 1,135,548	$ 1,034,149	$ 1,030,344
Rental Expenses: % Of Grand Totals		100.00%	100.00%	100.00%
Net Operating Income		$ 1,929,527	$ 1,814,288	$ 1,694,075
Net Operating Income: % of Grand Totals		100.00%	100.00%	100.00%
Company's Share of Net Operating Income: % of Grand Totals		100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 1,005,156	$ 930,560	$ 897,915
Rental Revenue: Residential	[1]	15,086	13,397	13,616
Rental Revenue Total	[1]	$ 1,059,724	$ 957,566	$ 919,009
Rental Revenue: % of Grand Totals		34.58%	33.62%	33.73%
Rental Expenses: Class A Office		$ 360,218	$ 322,298	$ 318,509
Rental Expenses: Residential		5,961	5,811	5,378
Rental Expenses: Total		$ 393,657	$ 341,107	$ 337,023
Rental Expenses: % Of Grand Totals		34.66%	32.98%	32.71%
Net Operating Income		$ 666,067	$ 616,459	$ 581,986
Net Operating Income: % of Grand Totals		34.52%	33.98%	34.35%
Company's Share of Net Operating Income: % of Grand Totals		34.75%	33.98%	33.88%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 0	$ 0	$ 0
Rental Revenue: Residential	[1]	0	0	0
Rental Revenue Total	[1]	$ 0	$ 0	$ 0
Rental Revenue: % of Grand Totals		0.00%	0.00%	0.00%
Rental Expenses: Class A Office		$ 0	$ 0	$ 0
Rental Expenses: Residential		0	0	0
Rental Expenses: Total		$ 0	$ 0	$ 0
Rental Expenses: % Of Grand Totals		0.00%	0.00%	0.00%
Net Operating Income		$ 0	$ 0	$ 0
Net Operating Income: % of Grand Totals		0.00%	0.00%	0.00%
Company's Share of Net Operating Income: % of Grand Totals		2.81%	2.98%	3.56%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 1,031,479	$ 1,012,172	$ 935,966
Rental Revenue: Residential	[1]	0	0	0
Rental Revenue Total	[1]	$ 1,031,479	$ 1,012,172	$ 935,966
Rental Revenue: % of Grand Totals		33.65%	35.53%	34.36%
Rental Expenses: Class A Office		$ 391,293	$ 379,267	$ 384,753
Rental Expenses: Residential		0	0	0
Rental Expenses: Total		$ 391,293	$ 379,267	$ 384,753
Rental Expenses: % Of Grand Totals		34.46%	36.67%	37.35%
Net Operating Income		$ 640,186	$ 632,905	$ 551,213
Net Operating Income: % of Grand Totals		33.18%	34.88%	32.54%
Company's Share of Net Operating Income: % of Grand Totals		26.32%	28.18%	26.12%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 534,397	$ 508,620	$ 508,327
Rental Revenue: Residential	[1]	14,769	3,892	155
Rental Revenue Total	[1]	$ 549,166	$ 512,512	$ 508,482
Rental Revenue: % of Grand Totals		17.92%	17.99%	18.66%
Rental Expenses: Class A Office		$ 183,353	$ 168,040	$ 163,156
Rental Expenses: Residential		11,371	6,717	2,261
Rental Expenses: Total		$ 194,724	$ 174,757	$ 165,417
Rental Expenses: % Of Grand Totals		17.15%	16.90%	16.05%
Net Operating Income		$ 354,442	$ 337,755	$ 343,065
Net Operating Income: % of Grand Totals		18.37%	18.62%	20.25%
Company's Share of Net Operating Income: % of Grand Totals		19.49%	20.27%	22.02%
Seattle
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 31,978	$ 0
Rental Revenue: Residential	[1]	0	0
Rental Revenue Total	[1]	$ 31,978	$ 0
Rental Revenue: % of Grand Totals		1.04%	0.00%
Rental Expenses: Class A Office		$ 8,386	$ 0
Rental Expenses: Residential		0	0
Rental Expenses: Total		$ 8,386	$ 0
Rental Expenses: % Of Grand Totals		0.74%	0.00%
Net Operating Income		$ 23,592	$ 0
Net Operating Income: % of Grand Totals		1.22%	0.00%
Company's Share of Net Operating Income: % of Grand Totals		1.66%	0.14%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 365,402	$ 340,808	$ 336,587
Rental Revenue: Residential	[1]	27,326	25,379	24,375
Rental Revenue Total	[1]	$ 392,728	$ 366,187	$ 360,962
Rental Revenue: % of Grand Totals		12.81%	12.86%	13.25%
Rental Expenses: Class A Office		$ 135,237	$ 127,102	$ 132,051
Rental Expenses: Residential		12,251	11,916	11,100
Rental Expenses: Total		$ 147,488	$ 139,018	$ 143,151
Rental Expenses: % Of Grand Totals		12.99%	13.45%	13.89%
Net Operating Income		$ 245,240	$ 227,169	$ 217,811
Net Operating Income: % of Grand Totals		12.71%	12.52%	12.86%
Company's Share of Net Operating Income: % of Grand Totals		14.97%	14.45%	14.42%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	$ 39,482	$ 13,609	$ 7,478
Operating expense		27,478	12,998	13,136
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	39,482	13,609	7,478
Operating expense		27,478	12,998	13,136
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0	0
Operating expense		0	0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0	0
Operating expense		0	0	0
Hotel [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0	0
Operating expense		0	0	0
Hotel [Member] | Seattle
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Operating expense		0	0
Hotel [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0	0
Operating expense		0	0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(191,812)	(186,304)	(162,887)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(45,822)	(43,232)	(41,849)
Noncontrolling interest - property partnerships [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0	0	0
Noncontrolling interest - property partnerships [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(145,990)	(143,072)	(121,038)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0	0	0
Noncontrolling interest - property partnerships [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income		0	0
Noncontrolling interest - property partnerships [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0	0	0
Unconsolidated Joint Ventures [Member]
Segment Reporting Information [Line Items]
Net Operating Income		146,081	107,756	94,943
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		34,233	16,551	10,765
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		53,023	51,641	57,907
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		1,594	(664)	(5,326)
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		12,785	14,152	14,928
Unconsolidated Joint Ventures [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income		7,690	2,498
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		36,756	23,578	16,669
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income		1,883,796	1,735,740	1,626,131
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		654,478	589,778	550,902
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		53,023	51,641	57,907
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		495,790	489,169	424,849
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		367,227	351,907	357,993
Company's Share [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income		31,282	2,498
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		$ 281,996	$ 250,747	$ 234,480

[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.