Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 2,792,160	$ 2,678,795	$ 2,824,634
Rental Revenue: Residential	42,668	38,146	36,914
Rental Revenue Total	$ 2,848,437	$ 2,724,419	$ 2,910,137
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 996,707	$ 998,469	$ 1,034,025
Rental Expenses: Residential	24,444	18,739	15,985
Rental Expenses: Total	$ 1,034,149	$ 1,030,344	$ 1,084,014
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net Operating Income	$ 1,814,288	$ 1,694,075	$ 1,826,123
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%
Segment Reconciling Items
Segment Reporting Information [Line Items]
Other revenue	$ (12,487)	$ (11,626)	$ (10,386)
Operating expense	(12,487)	(11,626)	(10,386)
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	930,560	897,915	895,098
Rental Revenue: Residential	13,397	13,616	13,786
Rental Revenue Total	$ 957,566	$ 919,009	$ 957,473
Rental Revenue: % of Grand Totals	33.62%	33.73%	32.90%
Rental Expenses: Class A Office	$ 322,298	$ 318,509	$ 322,282
Rental Expenses: Residential	5,811	5,378	5,071
Rental Expenses: Total	$ 341,107	$ 337,023	$ 361,357
Rental Expenses: % Of Grand Totals	32.98%	32.71%	33.34%
Net Operating Income	$ 616,459	$ 581,986	$ 596,116
Net Operating Income: % of Grand Totals	33.98%	34.35%	32.64%
Company's Share of Net Operating Income: % of Grand Totals	33.98%	33.88%	32.27%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 0	$ 0	$ 0
Rental Revenue: Residential	0	0	0
Rental Revenue Total	$ 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	2.98%	3.56%	3.53%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 1,012,172	$ 935,966	$ 1,011,912
Rental Revenue: Residential	0	0	0
Rental Revenue Total	$ 1,012,172	$ 935,966	$ 1,011,912
Rental Revenue: % of Grand Totals	35.53%	34.36%	34.78%
Rental Expenses: Class A Office	$ 379,267	$ 384,753	$ 389,532
Rental Expenses: Residential	0	0	0
Rental Expenses: Total	$ 379,267	$ 384,753	$ 389,532
Rental Expenses: % Of Grand Totals	36.67%	37.35%	35.93%
Net Operating Income	$ 632,905	$ 551,213	$ 622,380
Net Operating Income: % of Grand Totals	34.88%	32.54%	34.09%
Company's Share of Net Operating Income: % of Grand Totals	28.18%	26.12%	27.77%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 508,620	$ 508,327	$ 533,189
Rental Revenue: Residential	3,892	155	0
Rental Revenue Total	$ 512,512	$ 508,482	$ 533,189
Rental Revenue: % of Grand Totals	17.99%	18.66%	18.32%
Rental Expenses: Class A Office	$ 168,040	$ 163,156	$ 177,994
Rental Expenses: Residential	6,717	2,261	0
Rental Expenses: Total	$ 174,757	$ 165,417	$ 177,994
Rental Expenses: % Of Grand Totals	16.90%	16.05%	16.42%
Net Operating Income	$ 337,755	$ 343,065	$ 355,195
Net Operating Income: % of Grand Totals	18.62%	20.25%	19.45%
Company's Share of Net Operating Income: % of Grand Totals	20.27%	22.02%	20.39%
Seattle
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 0
Rental Revenue: Residential	0
Rental Revenue Total	$ 0
Rental Revenue: % of Grand Totals	0.00%
Rental Expenses: Class A Office	$ 0
Rental Expenses: Residential	0
Rental Expenses: Total	$ 0
Rental Expenses: % Of Grand Totals	0.00%
Net Operating Income	$ 0
Net Operating Income: % of Grand Totals	0.00%
Company's Share of Net Operating Income: % of Grand Totals	0.14%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 340,808	$ 336,587	$ 384,435
Rental Revenue: Residential	25,379	24,375	23,128
Rental Revenue Total	$ 366,187	$ 360,962	$ 407,563
Rental Revenue: % of Grand Totals	12.86%	13.25%	14.00%
Rental Expenses: Class A Office	$ 127,102	$ 132,051	$ 144,217
Rental Expenses: Residential	11,916	11,100	10,914
Rental Expenses: Total	$ 139,018	$ 143,151	$ 155,131
Rental Expenses: % Of Grand Totals	13.45%	13.89%	14.31%
Net Operating Income	$ 227,169	$ 217,811	$ 252,432
Net Operating Income: % of Grand Totals	12.52%	12.86%	13.82%
Company's Share of Net Operating Income: % of Grand Totals	14.45%	14.42%	16.04%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	$ 13,609	$ 7,478	$ 48,589
Operating expense	12,998	13,136	34,004
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	13,609	7,478	48,589
Operating expense	12,998	13,136	34,004
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0	0
Operating expense	0	0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0	0
Operating expense	0	0	0
Hotel [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0	0
Operating expense	0	0	0
Hotel [Member] | Seattle
Segment Reporting Information [Line Items]
Other revenue	0
Operating expense	0
Hotel [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0	0
Operating expense	0	0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(186,304)	(162,887)	(183,989)
Noncontrolling interest - property partnerships [Member] | Segment Reconciling Items
Segment Reporting Information [Line Items]
Net Operating Income	(186,304)	(162,887)	(183,989)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(43,232)	(41,849)	(40,109)
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	(143,072)	(121,038)	(143,432)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	0	0	(448)
Noncontrolling interest - property partnerships [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income	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 Reconciling Items
Segment Reporting Information [Line Items]
Net Operating Income	(107,756)	(94,943)	(97,716)
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	16,551	10,765	5,494
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income	51,641	57,907	61,338
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(664)	(5,326)	4,174
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	14,152	14,928	0
Unconsolidated Joint Ventures [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income	2,498
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income	23,578	16,669	26,710
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income	1,735,740	1,626,131	1,739,850
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	589,778	550,902	561,501
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income	51,641	57,907	61,338
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income	489,169	424,849	483,122
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	351,907	357,993	354,747
Company's Share [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income	2,498
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income	$ 250,747	$ 234,480	$ 279,142