Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 2,345,175	$ 2,240,119	$ 1,987,671
Rental Revenue: Office/Technical	58,163	61,985	55,525
Rental Revenue: Residential	18,883	26,193	22,318
Rental Revenue: Hotel	46,046	43,385	40,330
Rental Revenue Total	$ 2,468,267	$ 2,371,682	$ 2,105,844
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	$ 848,763	$ 802,902	$ 716,049
Rental Expenses: Office/Technical	15,262	16,466	14,777
Rental Expenses: Residential	8,227	15,922	12,130
Rental Expenses: Hotel	32,084	29,236	28,447
Rental Expenses: Total	$ 904,336	$ 864,526	$ 771,403
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net operating Income	$ 1,563,931	$ 1,507,156	$ 1,334,441
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 692,419	$ 692,116	$ 665,991
Rental Revenue: Office/Technical	23,827	23,801	22,617
Rental Revenue: Residential	4,801	4,528	4,395
Rental Revenue: Hotel	46,046	43,385	40,330
Rental Revenue Total	$ 767,093	$ 763,830	$ 733,333
Rental Revenue: % of Grand Totals	31.08%	32.21%	34.82%
Rental Expenses: Class A Office	$ 280,307	$ 270,947	$ 259,997
Rental Expenses: Office/Technical	7,034	7,173	6,879
Rental Expenses: Residential	2,006	1,957	1,823
Rental Expenses: Hotel	32,084	29,236	28,447
Rental Expenses: Total	$ 321,431	$ 309,313	$ 297,146
Rental Expenses: % Of Grand Totals	35.54%	35.78%	38.52%
Net operating Income	$ 445,662	$ 454,517	$ 436,187
Net operating Income: % of Grand Totals	28.50%	30.16%	32.69%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 1,000,030	$ 928,692	$ 725,566
Rental Revenue: Office/Technical	0	0	0
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 1,000,030	$ 928,692	$ 725,566
Rental Revenue: % of Grand Totals	40.52%	39.16%	34.46%
Rental Expenses: Class A Office	$ 346,897	$ 315,330	$ 251,640
Rental Expenses: Office/Technical	0	0	0
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 346,897	$ 315,330	$ 251,640
Rental Expenses: % Of Grand Totals	38.36%	36.47%	32.62%
Net operating Income	$ 653,133	$ 613,362	$ 473,926
Net operating Income: % of Grand Totals	41.76%	40.70%	35.51%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 280,005	$ 237,381	$ 214,755
Rental Revenue: Office/Technical	22,429	23,840	17,259
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 302,434	$ 261,221	$ 232,014
Rental Revenue: % of Grand Totals	12.25%	11.01%	11.02%
Rental Expenses: Class A Office	$ 94,268	$ 85,178	$ 77,905
Rental Expenses: Office/Technical	3,938	4,955	3,708
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 98,206	$ 90,133	$ 81,613
Rental Expenses: % Of Grand Totals	10.86%	10.43%	10.58%
Net operating Income	$ 204,228	$ 171,088	$ 150,401
Net operating Income: % of Grand Totals	13.06%	11.35%	11.27%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 372,721	$ 381,930	$ 381,359
Rental Revenue: Office/Technical	11,907	14,344	15,649
Rental Revenue: Residential	14,082	21,665	17,923
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	$ 398,710	$ 417,939	$ 414,931
Rental Revenue: % of Grand Totals	16.15%	17.62%	19.70%
Rental Expenses: Class A Office	$ 127,291	$ 131,447	$ 126,507
Rental Expenses: Office/Technical	4,290	4,338	4,190
Rental Expenses: Residential	6,221	13,965	10,307
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	$ 137,802	$ 149,750	$ 141,004
Rental Expenses: % Of Grand Totals	15.24%	17.32%	18.28%
Net operating Income	$ 260,908	$ 268,189	$ 273,927
Net operating Income: % of Grand Totals	16.68%	17.79%	20.53%