Segment Reporting (Schedule Of Segment Reporting By Geographic Area And Property Type) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Class A Office	$ 438,306	$ 405,005	$ 854,347	$ 796,662
Office/Technical	9,942	10,287	19,685	20,538
Residential	5,036	576	9,517	576
Hotel	10,049	8,904	16,865	14,852
Total	463,333	424,772	900,414	832,628
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Class A Office	156,625	140,120	307,785	276,021
Office/Technical	2,654	2,790	5,225	5,780
Residential	2,574	723	5,289	723
Hotel	6,616	6,281	12,715	12,020
Total	168,469	149,914	331,014	294,544
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Net operating income	294,864		569,400
% of Grand Totals	100.00%	100.00%	100.00%	100.00%
Segment, Continuing Operations [Member]
Net operating income		274,858		538,084
Boston [Member]
Class A Office	157,642	135,505	298,650	262,302
Office/Technical	5,744	6,337	11,343	12,596
Residential	1,013		1,896
Hotel	10,049	8,904	16,865	14,852
Total	174,448	150,746	328,754	289,750
% of Grand Totals	37.65%	35.49%	36.51%	34.80%
Class A Office	60,504	50,772	118,337	98,542
Office/Technical	1,652	1,779	3,152	3,647
Residential	367		757
Hotel	6,616	6,281	12,715	12,020
Total	69,139	58,832	134,961	114,209
% of Grand Totals	41.04%	39.25%	40.77%	38.77%
Net operating income	105,309	91,914	193,793	175,541
% of Grand Totals	35.71%	33.44%	34.03%	32.62%
New York [Member]
Class A Office	122,657		240,246	226,414
Total	122,657	113,407	240,246	226,414
% of Grand Totals	26.47%	26.70%	26.68%	27.19%
Class A Office	38,936	37,378	77,443	75,092
Total	38,936	37,378	77,443	75,092
% of Grand Totals	23.11%	24.93%	23.39%	25.50%
Net operating income	83,721		162,803	151,322
% of Grand Totals	28.39%	27.66%	28.59%	28.12%
Princeton [Member]
Class A Office	15,617	15,811	30,467	32,090
Total	15,617	15,811	30,467	32,090
% of Grand Totals	3.37%	3.72%	3.38%	3.85%
Class A Office	7,691	7,487	15,049	15,402
Total	7,691	7,487	15,049	15,402
% of Grand Totals	4.57%	4.99%	4.55%	5.23%
Net operating income	7,926	8,324	15,418	16,688
% of Grand Totals	2.69%	3.03%	2.71%	3.10%
San Francisco [Member]
Class A Office	53,650	54,168	105,847	107,061
Office/Technical	144		196
Total	53,794	54,168	106,043	107,061
% of Grand Totals	11.61%	12.75%	11.78%	12.86%
Class A Office	21,188	20,118	41,209	39,452
Office/Technical	38		62
Total	21,226	20,118	41,271	39,452
% of Grand Totals	12.60%	13.42%	12.47%	13.39%
Net operating income	32,568	34,050	64,772	67,609
% of Grand Totals	11.05%	12.39%	11.38%	12.57%
Washington, DC [Member]
Class A Office	88,740	86,114	179,137	168,795
Office/Technical	4,054	3,950	8,146	7,942
Residential	4,023	576	7,621	576
Total	96,817	90,640	194,904	177,313
% of Grand Totals	20.90%	21.34%	21.65%	21.30%
Class A Office	28,306	24,365	55,747	47,533
Office/Technical	964	1,011	2,011	2,133
Residential	2,207	723	4,532	723
Total	31,477	26,099	62,290	50,389
% of Grand Totals	18.68%	17.41%	18.82%	17.11%
Net operating income	$ 65,340	$ 64,541	$ 132,614	$ 126,924
% of Grand Totals	22.16%	23.48%	23.29%	23.59%