Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 614,380	$ 646,490
Rental Revenue: Residential	3,956	4,049
Rental Revenue: Hotel	7,420	8,757
Total revenue	$ 625,756	$ 659,296
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Office	$ 226,736	$ 217,572
Rental Expenses: Residential	1,551	1,600
Rental Expenses: Hotel	7,091	7,634
Rental Expenses: Total	$ 235,378	$ 226,806
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net operating Income	$ 390,378	$ 432,490
Net operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 185,436	$ 177,827
Rental Revenue: Residential	1,139	1,171
Rental Revenue: Hotel	7,420	8,757
Total revenue	$ 193,995	$ 187,755
Rental Revenue: % of Grand Totals	31.00%	28.48%
Rental Expenses: Office	$ 75,256	$ 70,687
Rental Expenses: Residential	495	520
Rental Expenses: Hotel	7,091	7,634
Rental Expenses: Total	$ 82,842	$ 78,841
Rental Expenses: % Of Grand Totals	35.19%	34.76%
Net operating Income	$ 111,153	$ 108,914
Net operating Income: % of Grand Totals	28.47%	25.18%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 241,570	$ 291,858
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 241,570	$ 291,858
Rental Revenue: % of Grand Totals	38.60%	44.27%
Rental Expenses: Office	$ 91,684	$ 88,798
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 91,684	$ 88,798
Rental Expenses: % Of Grand Totals	38.95%	39.15%
Net operating Income	$ 149,886	$ 203,060
Net operating Income: % of Grand Totals	38.40%	46.95%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 84,641	$ 76,317
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 84,641	$ 76,317
Rental Revenue: % of Grand Totals	13.53%	11.57%
Rental Expenses: Office	$ 24,474	$ 23,905
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 24,474	$ 23,905
Rental Expenses: % Of Grand Totals	10.40%	10.54%
Net operating Income	$ 60,167	$ 52,412
Net operating Income: % of Grand Totals	15.41%	12.12%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 102,733	$ 100,488
Rental Revenue: Residential	2,817	2,878
Rental Revenue: Hotel	0	0
Total revenue	$ 105,550	$ 103,366
Rental Revenue: % of Grand Totals	16.87%	15.68%
Rental Expenses: Office	$ 35,322	$ 34,182
Rental Expenses: Residential	1,056	1,080
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 36,378	$ 35,262
Rental Expenses: % Of Grand Totals	15.46%	15.55%
Net operating Income	$ 69,172	$ 68,104
Net operating Income: % of Grand Totals	17.72%	15.75%