Segment Reporting (Schedule Of Segment Reporting By Geographic Area And Property Type) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Class A Office	$ 438,239	$ 420,332	$ 1,292,586	$ 1,216,993
Office/Technical	9,786	10,695	29,471	31,234
Residential	5,496	2,453	15,013	3,029
Hotel	9,359	8,045	26,224	22,897
Total	462,880	441,525	1,363,294	1,274,153
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Class A Office	163,334	148,374	471,119	424,395
Office/Technical	2,751	2,984	7,976	8,764
Residential	2,854	2,418	8,143	3,141
Hotel	6,886	6,032	19,601	18,052
Total	175,825	159,808	506,839	454,352
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating income	287,055	281,717	856,455	819,801
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Boston [Member]
Class A Office	161,479	141,024	460,127	403,325
Office/Technical	5,539	6,460	16,882	19,057
Residential	995	284	2,891	284
Hotel	9,359	8,045	26,224	22,897
Total	177,372	155,813	506,124	445,563
% of Grand Totals	38.32%	35.29%	37.13%	34.97%
Class A Office	65,261	55,308	183,601	153,849
Office/Technical	1,758	1,876	4,910	5,522
Residential	490	270	1,247	270
Hotel	6,886	6,032	19,601	18,052
Total	74,395	63,486	209,359	177,693
% of Grand Totals	42.31%	39.73%	41.31%	39.11%
Net operating income	102,977	92,327	296,765	267,870
% of Grand Totals	35.87%	32.77%	34.65%	32.67%
New York [Member]
Class A Office	120,270	115,284	360,515	341,698
Total	120,270	115,284	360,515	341,698
% of Grand Totals	25.98%	26.11%	26.44%	26.82%
Class A Office	41,088	39,331	118,531	114,423
Total	41,088	39,331	118,531	114,423
% of Grand Totals	23.37%	24.61%	23.38%	25.18%
Net operating income	79,182	75,953	241,984	227,275
% of Grand Totals	27.59%	26.96%	28.26%	27.72%
Princeton [Member]
Class A Office	15,234	15,495	45,701	47,585
Total	15,234	15,495	45,701	47,585
% of Grand Totals	3.29%	3.51%	3.35%	3.73%
Class A Office	7,185	7,189	22,234	22,591
Total	7,185	7,189	22,234	22,591
% of Grand Totals	4.09%	4.50%	4.39%	4.97%
Net operating income	8,049	8,306	23,467	24,994
% of Grand Totals	2.80%	2.95%	2.74%	3.05%
San Francisco [Member]
Class A Office	54,137	53,978	159,984	161,039
Office/Technical	148		344
Total	54,285	53,978	160,328	161,039
% of Grand Totals	11.73%	12.23%	11.76%	12.64%
Class A Office	21,684	20,788	62,892	60,241
Office/Technical	52		114
Total	21,736	20,788	63,006	60,241
% of Grand Totals	12.36%	13.01%	12.43%	13.26%
Net operating income	32,549	33,190	97,322	100,798
% of Grand Totals	11.34%	11.78%	11.36%	12.30%
Washington, DC [Member]
Class A Office	87,119	94,551	266,259	263,346
Office/Technical	4,099	4,235	12,245	12,177
Residential	4,501	2,169	12,122	2,745
Total	95,719	100,955	290,626	278,268
% of Grand Totals	20.68%	22.86%	21.32%	21.84%
Class A Office	28,116	25,758	83,861	73,291
Office/Technical	941	1,108	2,952	3,242
Residential	2,364	2,148	6,896	2,871
Total	31,421	29,014	93,709	79,404
% of Grand Totals	17.87%	18.15%	18.49%	17.48%
Net operating income	$ 64,298	$ 71,941	$ 196,917	$ 198,864
% of Grand Totals	22.40%	25.54%	22.99%	24.26%