Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$ 475,965	$ 436,758	$ 926,527	$ 851,266
Rental Revenue: Office/Technical	14,896	9,942	24,743	19,685
Rental Revenue: Residential	5,484	5,036	11,062	9,517
Rental Revenue: Hotel	11,118	10,049	19,409	16,865
Total revenue	507,463	461,785	981,741	897,333
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	173,237	155,944	340,298	306,500
Rental Expenses: Office/Technical	3,770	2,654	6,596	5,225
Rental Expenses: Residential	2,830	2,574	5,563	5,289
Rental Expenses: Hotel	7,335	6,616	14,379	12,715
Rental Expenses: Total	187,172	167,788	366,836	329,729
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating Income	320,291	293,997	614,905	567,604
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	164,855	157,642	326,358	298,650
Rental Revenue: Office/Technical	5,538	5,744	11,191	11,343
Rental Revenue: Residential	1,065	1,013	2,132	1,896
Rental Revenue: Hotel	11,118	10,049	19,409	16,865
Total revenue	182,576	174,448	359,090	328,754
Rental Revenue: % of Grand Totals	35.98%	37.78%	36.58%	36.64%
Rental Expenses: Class A Office	63,181	60,504	128,913	118,337
Rental Expenses: Office/Technical	1,627	1,652	3,414	3,152
Rental Expenses: Residential	407	367	850	757
Rental Expenses: Hotel	7,335	6,616	14,379	12,715
Rental Expenses: Total	72,550	69,139	147,556	134,961
Rental Expenses: % Of Grand Totals	38.76%	41.21%	40.22%	40.93%
Net operating Income	110,026	105,309	211,534	193,793
Net operating Income: % of Grand Totals	34.35%	35.82%	34.40%	34.14%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	143,305	122,657	265,401	240,246
Rental Revenue: Office/Technical	0	0	0	0
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	143,305	122,657	265,401	240,246
Rental Revenue: % of Grand Totals	28.24%	26.56%	27.03%	26.77%
Rental Expenses: Class A Office	48,205	38,936	90,092	77,443
Rental Expenses: Office/Technical	0	0	0	0
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	48,205	38,936	90,092	77,443
Rental Expenses: % Of Grand Totals	25.75%	23.21%	24.56%	23.49%
Net operating Income	95,100	83,721	175,309	162,803
Net operating Income: % of Grand Totals	29.69%	28.48%	28.51%	28.68%
Princeton [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	14,853	15,617	31,280	30,467
Rental Revenue: Office/Technical	0	0	0	0
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	14,853	15,617	31,280	30,467
Rental Revenue: % of Grand Totals	2.93%	3.38%	3.19%	3.40%
Rental Expenses: Class A Office	7,667	7,691	15,314	15,049
Rental Expenses: Office/Technical	0	0	0	0
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	7,667	7,691	15,314	15,049
Rental Expenses: % Of Grand Totals	4.10%	4.58%	4.17%	4.56%
Net operating Income	7,186	7,926	15,966	15,418
Net operating Income: % of Grand Totals	2.24%	2.70%	2.60%	2.72%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	53,331	52,102	106,002	102,766
Rental Revenue: Office/Technical	5,327	144	5,478	196
Rental Revenue: Residential	0	0	0	0
Rental Revenue: Hotel	0	0	0	0
Total revenue	58,658	52,246	111,480	102,962
Rental Revenue: % of Grand Totals	11.56%	11.31%	11.35%	11.47%
Rental Expenses: Class A Office	21,264	20,507	41,136	39,924
Rental Expenses: Office/Technical	1,116	38	1,151	62
Rental Expenses: Residential	0	0	0	0
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	22,380	20,545	42,287	39,986
Rental Expenses: % Of Grand Totals	11.96%	12.24%	11.53%	12.13%
Net operating Income	36,278	31,701	69,193	62,976
Net operating Income: % of Grand Totals	11.33%	10.78%	11.25%	11.10%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	99,621	88,740	197,486	179,137
Rental Revenue: Office/Technical	4,031	4,054	8,074	8,146
Rental Revenue: Residential	4,419	4,023	8,930	7,621
Rental Revenue: Hotel	0	0	0	0
Total revenue	108,071	96,817	214,490	194,904
Rental Revenue: % of Grand Totals	21.29%	20.97%	21.85%	21.72%
Rental Expenses: Class A Office	32,920	28,306	64,843	55,747
Rental Expenses: Office/Technical	1,027	964	2,031	2,011
Rental Expenses: Residential	2,423	2,207	4,713	4,532
Rental Expenses: Hotel	0	0	0	0
Rental Expenses: Total	36,370	31,477	71,587	62,290
Rental Expenses: % Of Grand Totals	19.43%	18.76%	19.52%	18.89%
Net operating Income	$ 71,701	$ 65,340	$ 142,903	$ 132,614
Net operating Income: % of Grand Totals	22.39%	22.22%	23.24%	23.36%