Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 636,600	$ 614,380
Rental Revenue: Residential	4,159	3,956
Rental Revenue: Hotel	9,102	7,420
Total revenue	$ 649,861	$ 625,756
Rental Revenue: % of Grand Totals	100.00%	100.00%
Rental Expenses: Office	$ 238,057	$ 226,736
Rental Expenses: Residential	2,272	1,551
Rental Expenses: Hotel	8,073	7,091
Rental Expenses: Total	$ 248,402	$ 235,378
Rental Expenses: % Of Grand Totals	100.00%	100.00%
Net operating Income	$ 401,459	$ 390,378
Net operating Income: % of Grand Totals	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 204,997	$ 185,436
Rental Revenue: Residential	1,152	1,139
Rental Revenue: Hotel	9,102	7,420
Total revenue	$ 215,251	$ 193,995
Rental Revenue: % of Grand Totals	33.12%	31.00%
Rental Expenses: Office	$ 80,324	$ 75,256
Rental Expenses: Residential	514	495
Rental Expenses: Hotel	8,073	7,091
Rental Expenses: Total	$ 88,911	$ 82,842
Rental Expenses: % Of Grand Totals	35.79%	35.19%
Net operating Income	$ 126,340	$ 111,153
Net operating Income: % of Grand Totals	31.47%	28.47%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 242,398	$ 241,570
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 242,398	$ 241,570
Rental Revenue: % of Grand Totals	37.31%	38.60%
Rental Expenses: Office	$ 93,762	$ 91,684
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 93,762	$ 91,684
Rental Expenses: % Of Grand Totals	37.75%	38.95%
Net operating Income	$ 148,636	$ 149,886
Net operating Income: % of Grand Totals	37.02%	38.40%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 89,893	$ 84,641
Rental Revenue: Residential	0	0
Rental Revenue: Hotel	0	0
Total revenue	$ 89,893	$ 84,641
Rental Revenue: % of Grand Totals	13.83%	13.53%
Rental Expenses: Office	$ 27,628	$ 24,474
Rental Expenses: Residential	0	0
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 27,628	$ 24,474
Rental Expenses: % Of Grand Totals	11.12%	10.40%
Net operating Income	$ 62,265	$ 60,167
Net operating Income: % of Grand Totals	15.51%	15.41%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Office	$ 99,312	$ 102,733
Rental Revenue: Residential	3,007	2,817
Rental Revenue: Hotel	0	0
Total revenue	$ 102,319	$ 105,550
Rental Revenue: % of Grand Totals	15.74%	16.87%
Rental Expenses: Office	$ 36,343	$ 35,322
Rental Expenses: Residential	1,758	1,056
Rental Expenses: Hotel	0	0
Rental Expenses: Total	$ 38,101	$ 36,378
Rental Expenses: % Of Grand Totals	15.34%	15.46%
Net operating Income	$ 64,218	$ 69,172
Net operating Income: % of Grand Totals	16.00%	17.72%