Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 2,460,953	$ 2,403,338	$ 2,302,104
Rental Revenue: Residential	16,699	18,883	26,193
Rental Revenue: Hotel	44,884	46,046	43,385
Rental Revenue Total	$ 2,522,536	$ 2,468,267	$ 2,371,682
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 882,692	$ 864,025	$ 819,368
Rental Expenses: Residential	7,076	8,227	15,922
Rental Expenses: Hotel	31,466	32,084	29,236
Rental Expenses: Total	$ 921,234	$ 904,336	$ 864,526
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net operating Income	$ 1,601,302	$ 1,563,931	$ 1,507,156
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 727,265	$ 716,246	$ 715,917
Rental Revenue: Residential	4,812	4,801	4,528
Rental Revenue: Hotel	44,884	46,046	43,385
Rental Revenue Total	$ 776,961	$ 767,093	$ 763,830
Rental Revenue: % of Grand Totals	30.80%	31.08%	32.21%
Rental Expenses: Class A Office	$ 282,827	$ 287,341	$ 278,120
Rental Expenses: Residential	2,708	2,006	1,957
Rental Expenses: Hotel	31,466	32,084	29,236
Rental Expenses: Total	$ 317,001	$ 321,431	$ 309,313
Rental Expenses: % Of Grand Totals	34.41%	35.54%	35.78%
Net operating Income	$ 459,960	$ 445,662	$ 454,517
Net operating Income: % of Grand Totals	28.73%	28.50%	30.16%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 1,012,518	$ 1,000,030	$ 928,692
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 1,012,518	$ 1,000,030	$ 928,692
Rental Revenue: % of Grand Totals	40.14%	40.52%	39.16%
Rental Expenses: Class A Office	$ 363,188	$ 346,897	$ 315,330
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 363,188	$ 346,897	$ 315,330
Rental Expenses: % Of Grand Totals	39.42%	38.36%	36.47%
Net operating Income	$ 649,330	$ 653,133	$ 613,362
Net operating Income: % of Grand Totals	40.55%	41.76%	40.70%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 318,609	$ 302,434	$ 261,221
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 318,609	$ 302,434	$ 261,221
Rental Revenue: % of Grand Totals	12.63%	12.25%	11.01%
Rental Expenses: Class A Office	$ 100,787	$ 98,206	$ 90,133
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 100,787	$ 98,206	$ 90,133
Rental Expenses: % Of Grand Totals	10.94%	10.86%	10.43%
Net operating Income	$ 217,822	$ 204,228	$ 171,088
Net operating Income: % of Grand Totals	13.60%	13.06%	11.35%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 402,561	$ 384,628	$ 396,274
Rental Revenue: Residential	11,887	14,082	21,665
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 414,448	$ 398,710	$ 417,939
Rental Revenue: % of Grand Totals	16.43%	16.15%	17.62%
Rental Expenses: Class A Office	$ 135,890	$ 131,581	$ 135,785
Rental Expenses: Residential	4,368	6,221	13,965
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 140,258	$ 137,802	$ 149,750
Rental Expenses: % Of Grand Totals	15.23%	15.24%	17.32%
Net operating Income	$ 274,190	$ 260,908	$ 268,189
Net operating Income: % of Grand Totals	17.12%	16.68%	17.79%