Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 648,531	$ 629,542	$ 1,918,934	$ 1,875,879
Rental Revenue: Residential	6,320	4,295	15,278	12,461
Rental Revenue: Hotel	13,664	13,064	37,373	33,859
Total revenue	$ 668,515	$ 646,901	$ 1,971,585	$ 1,922,199
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Office	$ 244,265	$ 235,764	$ 717,199	$ 691,319
Rental Expenses: Residential	3,724	1,577	8,909	4,763
Rental Expenses: Hotel	8,828	8,447	25,642	23,942
Rental Expenses: Total	$ 256,817	$ 245,788	$ 751,750	$ 720,024
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	$ 411,698	$ 401,113	$ 1,219,835	$ 1,202,175
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 210,277	$ 196,687	$ 623,084	$ 573,883
Rental Revenue: Residential	1,905	1,228	4,252	3,520
Rental Revenue: Hotel	13,664	13,064	37,373	33,859
Total revenue	$ 225,846	$ 210,979	$ 664,709	$ 611,262
Rental Revenue: % of Grand Totals	33.78%	32.61%	33.71%	31.80%
Rental Expenses: Office	$ 78,089	$ 76,086	$ 235,560	$ 225,502
Rental Expenses: Residential	1,159	512	2,379	1,552
Rental Expenses: Hotel	8,828	8,447	25,642	23,942
Rental Expenses: Total	$ 88,076	$ 85,045	$ 263,581	$ 250,996
Rental Expenses: % Of Grand Totals	34.30%	34.60%	35.06%	34.86%
Net operating Income	$ 137,770	$ 125,934	$ 401,128	$ 360,266
Net operating Income: % of Grand Totals	33.46%	31.40%	32.88%	29.97%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 237,944	$ 242,071	$ 714,348	$ 735,485
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 237,944	$ 242,071	$ 714,348	$ 735,485
Rental Revenue: % of Grand Totals	35.60%	37.43%	36.23%	38.27%
Rental Expenses: Office	$ 96,795	$ 95,775	$ 282,395	$ 280,569
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 96,795	$ 95,775	$ 282,395	$ 280,569
Rental Expenses: % Of Grand Totals	37.68%	38.97%	37.57%	38.97%
Net operating Income	$ 141,149	$ 146,296	$ 431,953	$ 454,916
Net operating Income: % of Grand Totals	34.29%	36.47%	35.41%	37.84%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 102,035	$ 87,162	$ 285,410	$ 257,286
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 102,035	$ 87,162	$ 285,410	$ 257,286
Rental Revenue: % of Grand Totals	15.26%	13.47%	14.48%	13.38%
Rental Expenses: Office	$ 34,016	$ 26,792	$ 92,858	$ 77,204
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 34,016	$ 26,792	$ 92,858	$ 77,204
Rental Expenses: % Of Grand Totals	13.25%	10.90%	12.35%	10.72%
Net operating Income	$ 68,019	$ 60,370	$ 192,552	$ 180,082
Net operating Income: % of Grand Totals	16.52%	15.05%	15.79%	14.98%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 98,275	$ 103,622	$ 296,092	$ 309,225
Rental Revenue: Residential	4,415	3,067	11,026	8,941
Rental Revenue: Hotel	0	0	0	0
Total revenue	$ 102,690	$ 106,689	$ 307,118	$ 318,166
Rental Revenue: % of Grand Totals	15.36%	16.49%	15.58%	16.55%
Rental Expenses: Office	$ 35,365	$ 37,111	$ 106,386	$ 108,044
Rental Expenses: Residential	2,565	1,065	6,530	3,211
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	$ 37,930	$ 38,176	$ 112,916	$ 111,255
Rental Expenses: % Of Grand Totals	14.77%	15.53%	15.02%	15.45%
Net operating Income	$ 64,760	$ 68,513	$ 194,202	$ 206,911
Net operating Income: % of Grand Totals	15.73%	17.08%	15.92%	17.21%