Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 629,542	$ 602,138	$ 1,875,879	$ 1,849,745
Rental Revenue: Residential	4,295	4,372	12,461	12,509
Rental Revenue: Hotel	13,064	12,354	33,859	33,919
Total revenue	$ 646,901	$ 618,864	$ 1,922,199	$ 1,896,173
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Office	$ 235,764	$ 226,337	$ 691,319	$ 660,241
Rental Expenses: Residential	1,577	2,223	4,763	5,429
Rental Expenses: Hotel	8,447	8,118	23,942	23,730
Rental Expenses: Total	$ 245,788	$ 236,678	$ 720,024	$ 689,400
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	$ 401,113	$ 382,186	$ 1,202,175	$ 1,206,773
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 196,687	$ 183,975	$ 573,883	$ 540,850
Rental Revenue: Residential	1,228	1,227	3,520	3,578
Rental Revenue: Hotel	13,064	12,354	33,859	33,919
Total revenue	$ 210,979	$ 197,556	$ 611,262	$ 578,347
Rental Revenue: % of Grand Totals	32.61%	31.92%	31.80%	30.50%
Rental Expenses: Office	$ 76,086	$ 71,254	$ 225,502	$ 210,695
Rental Expenses: Residential	512	1,141	1,552	2,174
Rental Expenses: Hotel	8,447	8,118	23,942	23,730
Rental Expenses: Total	$ 85,045	$ 80,513	$ 250,996	$ 236,599
Rental Expenses: % Of Grand Totals	34.60%	34.02%	34.86%	34.32%
Net operating Income	$ 125,934	$ 117,043	$ 360,266	$ 341,748
Net operating Income: % of Grand Totals	31.40%	30.62%	29.97%	28.32%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 242,071	$ 237,262	$ 735,485	$ 773,077
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 242,071	$ 237,262	$ 735,485	$ 773,077
Rental Revenue: % of Grand Totals	37.43%	38.34%	38.27%	40.77%
Rental Expenses: Office	$ 95,775	$ 95,073	$ 280,569	$ 272,620
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 95,775	$ 95,073	$ 280,569	$ 272,620
Rental Expenses: % Of Grand Totals	38.97%	40.17%	38.97%	39.54%
Net operating Income	$ 146,296	$ 142,189	$ 454,916	$ 500,457
Net operating Income: % of Grand Totals	36.47%	37.21%	37.84%	41.47%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 87,162	$ 80,235	$ 257,286	$ 235,076
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 87,162	$ 80,235	$ 257,286	$ 235,076
Rental Revenue: % of Grand Totals	13.47%	12.96%	13.38%	12.40%
Rental Expenses: Office	$ 26,792	$ 26,037	$ 77,204	$ 75,412
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 26,792	$ 26,037	$ 77,204	$ 75,412
Rental Expenses: % Of Grand Totals	10.90%	11.00%	10.72%	10.94%
Net operating Income	$ 60,370	$ 54,198	$ 180,082	$ 159,664
Net operating Income: % of Grand Totals	15.05%	14.18%	14.98%	13.23%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 103,622	$ 100,666	$ 309,225	$ 300,742
Rental Revenue: Residential	3,067	3,145	8,941	8,931
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 106,689	$ 103,811	$ 318,166	$ 309,673
Rental Revenue: % of Grand Totals	16.49%	16.78%	16.55%	16.33%
Rental Expenses: Office	$ 37,111	$ 33,973	$ 108,044	$ 101,514
Rental Expenses: Residential	1,065	1,082	3,211	3,255
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 38,176	$ 35,055	$ 111,255	$ 104,769
Rental Expenses: % Of Grand Totals	15.53%	14.81%	15.45%	15.20%
Net operating Income	$ 68,513	$ 68,756	$ 206,911	$ 204,904
Net operating Income: % of Grand Totals	17.08%	17.99%	17.21%	16.98%