Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 693,580	$ 724,659
Rental Revenue: Residential	[1]	9,175	9,956
Rental Revenue Total	[1]	$ 703,387	$ 741,440
Rental Revenue: % of Grand Totals	[1]	100.00%	100.00%
Rental Expenses: Class A Office		$ 251,262	$ 258,902
Rental Expenses: Residential		6,127	4,064
Rental Expenses: Total		$ 259,440	$ 269,787
Rental Expenses: % Of Grand Totals		100.00%	100.00%
Net Operating Income		$ 443,947	$ 471,653
Net Operating Income: % of Grand Totals		100.00%	100.00%
Company's Share of Net Operating Income: % of Grand Totals		100.00%	100.00%
Total revenue		$ 713,695	$ 752,556
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	230,403	239,498
Rental Revenue: Residential	[1]	3,045	4,068
Rental Revenue Total	[1]	$ 234,080	$ 250,391
Rental Revenue: % of Grand Totals	[1]	33.28%	33.77%
Rental Expenses: Class A Office		$ 79,881	$ 82,545
Rental Expenses: Residential		1,455	1,340
Rental Expenses: Total		$ 83,387	$ 90,706
Rental Expenses: % Of Grand Totals		32.14%	33.62%
Net Operating Income		$ 150,693	$ 159,685
Net Operating Income: % of Grand Totals		33.94%	33.85%
Company's Share of Net Operating Income: % of Grand Totals		33.64%	33.60%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 0	$ 0
Rental Revenue: Residential	[1]	0	0
Rental Revenue Total	[1]	$ 0	$ 0
Rental Revenue: % of Grand Totals	[1]	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		3.34%	3.52%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 250,164	$ 255,286
Rental Revenue: Residential	[1]	0	0
Rental Revenue Total	[1]	$ 250,164	$ 255,286
Rental Revenue: % of Grand Totals	[1]	35.57%	34.43%
Rental Expenses: Class A Office		$ 99,385	$ 99,140
Rental Expenses: Residential		0	0
Rental Expenses: Total		$ 99,385	$ 99,140
Rental Expenses: % Of Grand Totals		38.31%	36.75%
Net Operating Income		$ 150,779	$ 156,146
Net Operating Income: % of Grand Totals		33.97%	33.11%
Company's Share of Net Operating Income: % of Grand Totals		27.30%	26.48%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 130,598	$ 136,739
Rental Revenue: Residential	[1]	321	0
Rental Revenue Total	[1]	$ 130,919	$ 136,739
Rental Revenue: % of Grand Totals	[1]	18.61%	18.44%
Rental Expenses: Class A Office		$ 40,249	$ 42,569
Rental Expenses: Residential		1,686	0
Rental Expenses: Total		$ 41,935	$ 42,569
Rental Expenses: % Of Grand Totals		16.16%	15.78%
Net Operating Income		$ 88,984	$ 94,170
Net Operating Income: % of Grand Totals		20.04%	19.97%
Company's Share of Net Operating Income: % of Grand Totals		21.79%	21.50%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 82,415	$ 93,136
Rental Revenue: Residential	[1]	5,809	5,888
Rental Revenue Total	[1]	$ 88,224	$ 99,024
Rental Revenue: % of Grand Totals	[1]	12.54%	13.36%
Rental Expenses: Class A Office		$ 31,747	$ 34,648
Rental Expenses: Residential		2,986	2,724
Rental Expenses: Total		$ 34,733	$ 37,372
Rental Expenses: % Of Grand Totals		13.39%	13.85%
Net Operating Income		$ 53,491	$ 61,652
Net Operating Income: % of Grand Totals		12.05%	13.07%
Company's Share of Net Operating Income: % of Grand Totals		13.93%	14.90%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	$ 632	$ 6,825
Operating expense		2,051	6,821
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	632	6,825
Operating expense		2,051	6,821
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Operating expense		0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Operating expense		0	0
Hotel [Member] | San Francisco [Member]
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
Operating expense		0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(44,376)	(47,661)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(10,224)	(10,663)
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		(34,152)	(36,998)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
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
Unconsolidated Joint Ventures [Member]
Segment Reporting Information [Line Items]
Net Operating Income		24,795	28,758
Total revenue	[2]	87,266	93,203
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		2,281	3,099
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		14,192	15,930
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(793)	756
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		3,480	3,159
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		5,635	5,814
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income		424,366	452,750
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		142,750	152,121
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		14,192	15,930
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		115,834	119,904
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		92,464	97,329
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		$ 59,126	$ 67,466

[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 $1.1 million and $9.7 million for the three months ended March 31, 2021 and 2020, respectively. For the three months ended March 31, 2021, write-offs of accounts receivable and accrued rent balances totaled approximately $0.3 million.