Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 696,442	$ 636,600
Rental Revenue: Residential	[1]	7,715	4,159
Rental Revenue: Hotel	[1]	8,938	9,102
Total revenue	[1]	$ 713,095	$ 649,861
Rental Revenue: % of Grand Totals		100.00%	100.00%
Rental Expenses: Office		$ 253,743	$ 238,057
Rental Expenses: Residential		3,774	2,272
Rental Expense: Hotel		7,863	8,073
Rental Expenses: Total		$ 265,380	$ 248,402
Rental Expenses: % Of Grand Totals		100.00%	100.00%
Net operating Income		$ 447,715	$ 401,459
Net operating Income: % of Grand Totals		100.00%	100.00%
Net operating income attributable to noncontrolling interests in property partnerships		$ (47,085)	$ (45,909)
Company's share of net operating income from unconsolidated joint ventures		25,349	16,060
Company's Share of Net Operating Income		$ 425,979	$ 371,610
Company's Share of Net Operating Income: % of Grand Totals		100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 217,411	$ 204,997
Rental Revenue: Residential	[1]	2,701	1,152
Rental Revenue: Hotel	[1]	8,938	9,102
Total revenue	[1]	$ 229,050	$ 215,251
Rental Revenue: % of Grand Totals		32.12%	33.12%
Rental Expenses: Office		$ 79,500	$ 80,324
Rental Expenses: Residential		1,206	514
Rental Expense: Hotel		7,863	8,073
Rental Expenses: Total		$ 88,569	$ 88,911
Rental Expenses: % Of Grand Totals		33.37%	35.79%
Net operating Income		$ 140,481	$ 126,340
Net operating Income: % of Grand Totals		31.38%	31.47%
Net operating income attributable to noncontrolling interests in property partnerships		$ (9,373)	$ (8,129)
Company's share of net operating income from unconsolidated joint ventures		772	534
Company's Share of Net Operating Income		$ 131,880	$ 118,745
Company's Share of Net Operating Income: % of Grand Totals		30.96%	31.96%
Los Angeles [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 0	$ 0
Rental Revenue: Residential	[1]	0	0
Rental Revenue: Hotel	[1]	0	0
Total revenue	[1]	$ 0	$ 0
Rental Revenue: % of Grand Totals		0.00%	0.00%
Rental Expenses: Office		$ 0	$ 0
Rental Expenses: Residential		0	0
Rental Expense: Hotel		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%
Net operating income attributable to noncontrolling interests in property partnerships		$ 0	$ 0
Company's share of net operating income from unconsolidated joint ventures		15,708	7,074
Company's Share of Net Operating Income		$ 15,708	$ 7,074
Company's Share of Net Operating Income: % of Grand Totals		3.69%	1.90%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 258,631	$ 242,398
Rental Revenue: Residential	[1]	0	0
Rental Revenue: Hotel	[1]	0	0
Total revenue	[1]	$ 258,631	$ 242,398
Rental Revenue: % of Grand Totals		36.27%	37.31%
Rental Expenses: Office		$ 96,971	$ 93,762
Rental Expenses: Residential		0	0
Rental Expense: Hotel		0	0
Rental Expenses: Total		$ 96,971	$ 93,762
Rental Expenses: % Of Grand Totals		36.54%	37.75%
Net operating Income		$ 161,660	$ 148,636
Net operating Income: % of Grand Totals		36.11%	37.02%
Net operating income attributable to noncontrolling interests in property partnerships		$ (37,264)	$ (37,946)
Company's share of net operating income from unconsolidated joint ventures		1,786	1,661
Company's Share of Net Operating Income		$ 126,182	$ 112,351
Company's Share of Net Operating Income: % of Grand Totals		29.62%	30.23%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 124,055	$ 89,893
Rental Revenue: Residential	[1]	0	0
Rental Revenue: Hotel	[1]	0	0
Total revenue	[1]	$ 124,055	$ 89,893
Rental Revenue: % of Grand Totals		17.40%	13.83%
Rental Expenses: Office		$ 41,125	$ 27,628
Rental Expenses: Residential		0	0
Rental Expense: Hotel		0	0
Rental Expenses: Total		$ 41,125	$ 27,628
Rental Expenses: % Of Grand Totals		15.50%	11.12%
Net operating Income		$ 82,930	$ 62,265
Net operating Income: % of Grand Totals		18.52%	15.51%
Net operating income attributable to noncontrolling interests in property partnerships		$ (448)	$ 166
Company's share of net operating income from unconsolidated joint ventures		0	0
Company's Share of Net Operating Income		$ 82,482	$ 62,431
Company's Share of Net Operating Income: % of Grand Totals		19.36%	16.80%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	[1]	$ 96,345	$ 99,312
Rental Revenue: Residential	[1]	5,014	3,007
Rental Revenue: Hotel	[1]	0	0
Total revenue	[1]	$ 101,359	$ 102,319
Rental Revenue: % of Grand Totals		14.21%	15.74%
Rental Expenses: Office		$ 36,147	$ 36,343
Rental Expenses: Residential		2,568	1,758
Rental Expense: Hotel		0	0
Rental Expenses: Total		$ 38,715	$ 38,101
Rental Expenses: % Of Grand Totals		14.59%	15.34%
Net operating Income		$ 62,644	$ 64,218
Net operating Income: % of Grand Totals		13.99%	16.00%
Net operating income attributable to noncontrolling interests in property partnerships		$ 0	$ 0
Company's share of net operating income from unconsolidated joint ventures		7,083	6,791
Company's Share of Net Operating Income		$ 69,727	$ 71,009
Company's Share of Net Operating Income: % of Grand Totals		16.37%	19.11%

[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.