Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 724,659	$ 696,442
Rental Revenue: Residential	[1]	9,956	7,715
Rental Revenue Total	[1]	$ 741,440	$ 713,095
Rental Revenue: % of Grand Totals	[1]	100.00%	100.00%
Rental Expenses: Class A Office		$ 258,902	$ 253,743
Rental Expenses: Residential		4,064	3,774
Rental Expenses: Total		$ 269,787	$ 265,380
Rental Expenses: % Of Grand Totals		100.00%	100.00%
Net Operating Income		$ 471,653	$ 447,715
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	[1]	$ 239,498	$ 217,411
Rental Revenue: Residential	[1]	4,068	2,701
Rental Revenue Total	[1]	$ 250,391	$ 229,050
Rental Revenue: % of Grand Totals	[1]	33.77%	32.12%
Rental Expenses: Class A Office		$ 82,545	$ 79,500
Rental Expenses: Residential		1,340	1,206
Rental Expenses: Total		$ 90,706	$ 88,569
Rental Expenses: % Of Grand Totals		33.62%	33.37%
Net Operating Income		$ 159,685	$ 140,481
Net Operating Income: % of Grand Totals		33.85%	31.38%
Company's Share of Net Operating Income: % of Grand Totals		33.60%	30.96%
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		3.52%	3.69%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 255,286	$ 258,631
Rental Revenue: Residential	[1]	0	0
Rental Revenue Total	[1]	$ 255,286	$ 258,631
Rental Revenue: % of Grand Totals	[1]	34.43%	36.27%
Rental Expenses: Class A Office		$ 99,140	$ 96,971
Rental Expenses: Residential		0	0
Rental Expenses: Total		$ 99,140	$ 96,971
Rental Expenses: % Of Grand Totals		36.75%	36.54%
Net Operating Income		$ 156,146	$ 161,660
Net Operating Income: % of Grand Totals		33.11%	36.11%
Company's Share of Net Operating Income: % of Grand Totals		26.48%	29.62%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 136,739	$ 124,055
Rental Revenue: Residential	[1]	0	0
Rental Revenue Total	[1]	$ 136,739	$ 124,055
Rental Revenue: % of Grand Totals	[1]	18.44%	17.40%
Rental Expenses: Class A Office		$ 42,569	$ 41,125
Rental Expenses: Residential		0	0
Rental Expenses: Total		$ 42,569	$ 41,125
Rental Expenses: % Of Grand Totals		15.78%	15.50%
Net Operating Income		$ 94,170	$ 82,930
Net Operating Income: % of Grand Totals		19.97%	18.52%
Company's Share of Net Operating Income: % of Grand Totals		21.50%	19.36%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	[1]	$ 93,136	$ 96,345
Rental Revenue: Residential	[1]	5,888	5,014
Rental Revenue Total	[1]	$ 99,024	$ 101,359
Rental Revenue: % of Grand Totals	[1]	13.36%	14.21%
Rental Expenses: Class A Office		$ 34,648	$ 36,147
Rental Expenses: Residential		2,724	2,568
Rental Expenses: Total		$ 37,372	$ 38,715
Rental Expenses: % Of Grand Totals		13.85%	14.59%
Net Operating Income		$ 61,652	$ 62,644
Net Operating Income: % of Grand Totals		13.07%	13.99%
Company's Share of Net Operating Income: % of Grand Totals		14.90%	16.37%
Hotel [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	$ 6,825	$ 8,938
Payroll and related costs from management services contracts		6,821	7,863
Hotel [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	6,825	8,938
Payroll and related costs from management services contracts		6,821	7,863
Hotel [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Payroll and related costs from management services contracts		0	0
Hotel [Member] | New York [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Payroll and related costs from management services contracts		0	0
Hotel [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Payroll and related costs from management services contracts		0	0
Hotel [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Other revenue	[1]	0	0
Payroll and related costs from management services contracts		0	0
Noncontrolling interest - property partnerships [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(47,661)	(47,085)
Noncontrolling interest - property partnerships [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		(10,663)	(9,373)
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		(36,998)	(37,264)
Noncontrolling interest - property partnerships [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		0	(448)
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		28,758	25,349
Unconsolidated Joint Ventures [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		3,099	772
Unconsolidated Joint Ventures [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		15,930	15,708
Unconsolidated Joint Ventures [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		756	1,786
Unconsolidated Joint Ventures [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		3,159	0
Unconsolidated Joint Ventures [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		5,814	7,083
Company's Share [Member]
Segment Reporting Information [Line Items]
Net Operating Income		452,750	425,979
Company's Share [Member] | Boston [Member]
Segment Reporting Information [Line Items]
Net Operating Income		152,121	131,880
Company's Share [Member] | Los Angeles [Member]
Segment Reporting Information [Line Items]
Net Operating Income		15,930	15,708
Company's Share [Member] | New York [Member]
Segment Reporting Information [Line Items]
Net Operating Income		119,904	126,182
Company's Share [Member] | San Francisco [Member]
Segment Reporting Information [Line Items]
Net Operating Income		97,329	82,482
Company's Share [Member] | Washington, DC [Member]
Segment Reporting Information [Line Items]
Net Operating Income		$ 67,466	$ 69,727

[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.