Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 726,888	$ 693,580
Rental Revenue: Residential	12,966	9,175
Rental Revenue Total	$ 744,411	$ 703,387
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Class A Office	$ 263,823	$ 251,262
Rental Expenses: Residential	6,432	6,127
Rental Expenses: Total	$ 275,095	$ 259,440
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net Operating Income	$ 469,316	$ 443,947
Net Operating Income: % of Grand Totals	100.00%	100.00%
Company's Share of Net Operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 242,078	$ 230,403
Rental Revenue: Residential	3,596	3,045
Rental Revenue Total	$ 250,231	$ 234,080
Rental Revenue: % of Grand Totals	33.61%	33.28%
Rental Expenses: Class A Office	$ 90,528	$ 79,881
Rental Expenses: Residential	1,437	1,455
Rental Expenses: Total	$ 96,805	$ 83,387
Rental Expenses: % Of Grand Totals	35.19%	32.14%
Net Operating Income	$ 153,426	$ 150,693
Net Operating Income: % of Grand Totals	32.69%	33.94%
Company's Share of Net Operating Income: % of Grand Totals	32.94%	33.64%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 0	$ 0
Rental Revenue: Residential	0	0
Rental Revenue Total	$ 0	$ 0
Rental Revenue: % of Grand Totals	0.00%	0.00%
Rental Expenses: Class A Office	$ 0	$ 0
Rental Expenses: Residential	0	0
Rental Expenses: Total	$ 0	$ 0
Rental Expenses: % Of Grand Totals	0.00%	0.00%
Net Operating Income	$ 0	$ 0
Net Operating Income: % of Grand Totals	0.00%	0.00%
Company's Share of Net Operating Income: % of Grand Totals	2.99%	3.34%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 256,870	$ 250,164
Rental Revenue: Residential	0	0
Rental Revenue Total	$ 256,870	$ 250,164
Rental Revenue: % of Grand Totals	34.51%	35.57%
Rental Expenses: Class A Office	$ 96,340	$ 99,385
Rental Expenses: Residential	0	0
Rental Expenses: Total	$ 96,340	$ 99,385
Rental Expenses: % Of Grand Totals	35.02%	38.31%
Net Operating Income	$ 160,530	$ 150,779
Net Operating Income: % of Grand Totals	34.21%	33.97%
Company's Share of Net Operating Income: % of Grand Totals	27.21%	27.30%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 132,375	$ 130,598
Rental Revenue: Residential	2,391	321
Rental Revenue Total	$ 134,766	$ 130,919
Rental Revenue: % of Grand Totals	18.10%	18.61%
Rental Expenses: Class A Office	$ 43,408	$ 40,249
Rental Expenses: Residential	1,868	1,686
Rental Expenses: Total	$ 45,276	$ 41,935
Rental Expenses: % Of Grand Totals	16.46%	16.16%
Net Operating Income	$ 89,490	$ 88,984
Net Operating Income: % of Grand Totals	19.07%	20.04%
Company's Share of Net Operating Income: % of Grand Totals	20.16%	21.79%
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	0.43%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 95,565	$ 82,415
Rental Revenue: Residential	6,979	5,809
Rental Revenue Total	$ 102,544	$ 88,224
Rental Revenue: % of Grand Totals	13.78%	12.54%
Rental Expenses: Class A Office	$ 33,547	$ 31,747
Rental Expenses: Residential	3,127	2,986
Rental Expenses: Total	$ 36,674	$ 34,733
Rental Expenses: % Of Grand Totals	13.33%	13.39%
Net Operating Income	$ 65,870	$ 53,491
Net Operating Income: % of Grand Totals	14.03%	12.05%
Company's Share of Net Operating Income: % of Grand Totals	16.27%	13.93%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	$ 4,557	$ 632
Operating expense	4,840	2,051
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	4,557	632
Operating expense	4,840	2,051
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0
Operating expense	0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0
Operating expense	0	0
Hotel [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Other revenue	0	0
Operating expense	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
Operating expense	0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(47,055)	(44,376)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(11,735)	(10,224)
Noncontrolling interest - property partnerships [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income	0	0
Noncontrolling interest - property partnerships [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(35,320)	(34,152)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	0	0
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
Unconsolidated Joint Ventures [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(37,321)	(24,795)
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	9,693	2,281
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income	13,757	14,192
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income	(156)	(793)
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	3,181	3,480
Unconsolidated Joint Ventures [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income	1,955
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income	8,891	5,635
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income	459,582	424,366
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income	151,384	142,750
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income	13,757	14,192
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income	125,054	115,834
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income	92,671	92,464
Company's Share [Member] | Seattle
Segment Reporting Information [Line Items]
Net Operating Income	1,955
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income	$ 74,761	$ 59,126