Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 3,119,999	$ 2,968,412	$ 2,792,160
Rental Revenue: Residential	[1]	47,592	57,181	42,668
Rental Revenue Total	[1]	$ 3,214,948	$ 3,065,075	$ 2,848,437
Rental Revenue: % of Grand Totals		100.00%	100.00%	100.00%
Rental Expenses: Class A Office		$ 1,160,697	$ 1,078,487	$ 996,707
Rental Expenses: Residential		23,250	29,583	24,444
Rental Expenses: Total		$ 1,216,172	$ 1,135,548	$ 1,034,149
Rental Expenses: % Of Grand Totals		100.00%	100.00%	100.00%
Net Operating Income		$ 1,998,776	$ 1,929,527	$ 1,814,288
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]	$ 1,093,840	$ 1,005,156	$ 930,560
Rental Revenue: Residential	[1]	16,452	15,086	13,397
Rental Revenue Total	[1]	$ 1,157,649	$ 1,059,724	$ 957,566
Rental Revenue: % of Grand Totals		36.01%	34.58%	33.62%
Rental Expenses: Class A Office		$ 393,339	$ 360,218	$ 322,298
Rental Expenses: Residential		6,378	5,961	5,811
Rental Expenses: Total		$ 431,942	$ 393,657	$ 341,107
Rental Expenses: % Of Grand Totals		35.51%	34.66%	32.98%
Net Operating Income		$ 725,707	$ 666,067	$ 616,459
Net Operating Income: % of Grand Totals		36.31%	34.52%	33.98%
Company's Share of Net Operating Income: % of Grand Totals		36.35%	34.75%	33.98%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 3,890	$ 0	$ 0
Rental Revenue: Residential	[1]	0	0	0
Rental Revenue Total	[1]	$ 3,890	$ 0	$ 0
Rental Revenue: % of Grand Totals		0.12%	0.00%	0.00%
Rental Expenses: Class A Office		$ 1,431	$ 0	$ 0
Rental Expenses: Residential		0	0	0
Rental Expenses: Total		$ 1,431	$ 0	$ 0
Rental Expenses: % Of Grand Totals		0.12%	0.00%	0.00%
Net Operating Income		$ 2,459	$ 0	$ 0
Net Operating Income: % of Grand Totals		0.12%	0.00%	0.00%
Company's Share of Net Operating Income: % of Grand Totals		2.61%	2.81%	2.98%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 1,053,615	$ 1,031,479	$ 1,012,172
Rental Revenue: Residential	[1]	0	0	0
Rental Revenue Total	[1]	$ 1,053,615	$ 1,031,479	$ 1,012,172
Rental Revenue: % of Grand Totals		32.77%	33.65%	35.53%
Rental Expenses: Class A Office		$ 415,831	$ 391,293	$ 379,267
Rental Expenses: Residential		0	0	0
Rental Expenses: Total		$ 415,831	$ 391,293	$ 379,267
Rental Expenses: % Of Grand Totals		34.19%	34.46%	36.67%
Net Operating Income		$ 637,784	$ 640,186	$ 632,905
Net Operating Income: % of Grand Totals		31.91%	33.18%	34.88%
Company's Share of Net Operating Income: % of Grand Totals		25.62%	26.32%	28.18%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 539,904	$ 534,397	$ 508,620
Rental Revenue: Residential	[1]	14,102	14,769	3,892
Rental Revenue Total	[1]	$ 554,006	$ 549,166	$ 512,512
Rental Revenue: % of Grand Totals		17.23%	17.92%	17.99%
Rental Expenses: Class A Office		$ 194,205	$ 183,353	$ 168,040
Rental Expenses: Residential		9,255	11,371	6,717
Rental Expenses: Total		$ 203,460	$ 194,724	$ 174,757
Rental Expenses: % Of Grand Totals		16.73%	17.15%	16.90%
Net Operating Income		$ 350,546	$ 354,442	$ 337,755
Net Operating Income: % of Grand Totals		17.54%	18.37%	18.62%
Company's Share of Net Operating Income: % of Grand Totals		18.66%	19.49%	20.27%
Seattle
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 63,830	$ 31,978	$ 0
Rental Revenue: Residential	[1]	0	0	0
Rental Revenue Total	[1]	$ 63,830	$ 31,978	$ 0
Rental Revenue: % of Grand Totals		1.99%	1.04%	0.00%
Rental Expenses: Class A Office		$ 12,387	$ 8,386	$ 0
Rental Expenses: Residential		0	0	0
Rental Expenses: Total		$ 12,387	$ 8,386	$ 0
Rental Expenses: % Of Grand Totals		1.02%	0.74%	0.00%
Net Operating Income		$ 51,443	$ 23,592	$ 0
Net Operating Income: % of Grand Totals		2.57%	1.22%	0.00%
Company's Share of Net Operating Income: % of Grand Totals		3.00%	1.66%	0.14%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 364,920	$ 365,402	$ 340,808
Rental Revenue: Residential	[1]	17,038	27,326	25,379
Rental Revenue Total	[1]	$ 381,958	$ 392,728	$ 366,187
Rental Revenue: % of Grand Totals		11.88%	12.81%	12.86%
Rental Expenses: Class A Office		$ 143,504	$ 135,237	$ 127,102
Rental Expenses: Residential		7,617	12,251	11,916
Rental Expenses: Total		$ 151,121	$ 147,488	$ 139,018
Rental Expenses: % Of Grand Totals		12.43%	12.99%	13.45%
Net Operating Income		$ 230,837	$ 245,240	$ 227,169
Net Operating Income: % of Grand Totals		11.55%	12.71%	12.52%
Company's Share of Net Operating Income: % of Grand Totals		13.76%	14.97%	14.45%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	$ 47,357	$ 39,482	$ 13,609
Operating expense		32,225	27,478	12,998
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	47,357	39,482	13,609
Operating expense		32,225	27,478	12,998
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	0
Operating expense		0	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		(194,365)	(191,812)	(186,304)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(45,730)	(45,822)	(43,232)
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		(148,635)	(145,990)	(143,072)
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	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				107,756
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		34,376	34,233	16,551
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		48,818	53,023	51,641
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		14,314	1,594	(664)
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		16,203	12,785	14,152
Unconsolidated Joint Ventures [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income		7,494	7,690	2,498
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		39,490	36,756	23,578
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income		1,965,106	1,883,796	1,735,740
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		714,353	654,478	589,778
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		51,277	53,023	51,641
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		503,463	495,790	489,169
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		366,749	367,227	351,907
Company's Share [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income		58,937	31,282	2,498
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		$ 270,327	$ 281,996	$ 250,747

[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.