Segment Information (Segment Reporting by Geographic Area and Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Rental Revenue: Class A	$ 406,264	$ 355,038	$ 799,213	$ 706,658
Rental Revenue: Office/Technical	11,609	11,548	23,159	23,152
Rental Revenue: Residential	576		576
Rental Revenue: Hotel	8,904	8,371	14,852	14,274
Rental Revenue: Total	427,353	374,957	837,800	744,084
Rental Revenue: % of Total	100.00%	100.00%	100.00%	100.00%
Real Estate Operating Expenses: Class A	140,729	120,208	277,098	241,752
Real Estate Operating Expenses: Office/Technical	3,423	3,076	7,025	6,517
Real Estate Operating Expenses: Residential	723		723
Real Estate Operating Expenses: Hotel	6,281	6,089	12,020	11,357
Real Estate Operating Expenses: Total	151,156	129,373	296,866	259,626
Real Estate Operating Expenses: % of Total	100.00%	100.00%	100.00%	100.00%
Net Operating Income	154,133	165,420	292,155	310,541
Net Operating Income: % of Total	100.00%	100.00%	100.00%	100.00%
Segment, Continuing Operations [Member]
Net Operating Income	276,197	245,584	540,934	484,458
Greater Boston [Member]
Rental Revenue: Class A	136,126	91,479	263,554	179,686
Rental Revenue: Office/Technical	7,659	7,597	15,217	15,181
Rental Revenue: Hotel	8,904	8,371	14,852	14,274
Rental Revenue: Total	152,689	107,447	293,623	209,141
Rental Revenue: % of Total	35.73%	28.66%	35.05%	28.11%
Real Estate Operating Expenses: Class A	51,070	33,919	98,967	67,399
Real Estate Operating Expenses: Office/Technical	2,412	2,107	4,892	4,373
Real Estate Operating Expenses: Hotel	6,281	6,089	12,020	11,357
Real Estate Operating Expenses: Total	59,763	42,115	115,879	83,129
Real Estate Operating Expenses: % of Total	39.54%	32.55%	39.03%	32.02%
Net Operating Income	92,926	65,332	177,744	126,012
Net Operating Income: % of Total	33.64%	26.60%	32.86%	26.01%
Greater Washington, DC [Member]
Rental Revenue: Class A	86,752	82,573	170,094	166,418
Rental Revenue: Office/Technical	3,950	3,951	7,942	7,971
Rental Revenue: Residential	576		576
Rental Revenue: Total	91,278	86,524	178,612	174,389
Rental Revenue: % of Total	21.36%	23.07%	21.32%	23.44%
Real Estate Operating Expenses: Class A	24,676	22,052	48,185	45,723
Real Estate Operating Expenses: Office/Technical	1,011	969	2,133	2,144
Real Estate Operating Expenses: Residential	723		723
Real Estate Operating Expenses: Total	26,410	23,021	51,041	47,867
Real Estate Operating Expenses: % of Total	17.47%	17.80%	17.19%	18.43%
Net Operating Income	64,868	63,503	127,571	126,522
Net Operating Income: % of Total	23.49%	25.86%	23.58%	26.12%
Midtown Manhattan [Member]
Rental Revenue: Class A	113,407	110,976	226,414	220,918
Rental Revenue: Total	113,407	110,976	226,414	220,918
Rental Revenue: % of Total	26.54%	29.60%	27.02%	29.69%
Real Estate Operating Expenses: Class A	37,378	36,824	75,092	74,269
Real Estate Operating Expenses: Total	37,378	36,824	75,092	74,269
Real Estate Operating Expenses: % of Total	24.73%	28.46%	25.30%	28.61%
Net Operating Income	76,029	74,152	151,322	146,649
Net Operating Income: % of Total	27.53%	30.19%	27.97%	30.27%
Greater San Francisco [Member]
Rental Revenue: Class A	54,168	53,484	107,061	106,631
Rental Revenue: Total	54,168	53,484	107,061	106,631
Rental Revenue: % of Total	12.67%	14.26%	12.78%	14.33%
Real Estate Operating Expenses: Class A	20,118	19,621	39,452	38,731
Real Estate Operating Expenses: Total	20,118	19,621	39,452	38,731
Real Estate Operating Expenses: % of Total	13.31%	15.17%	13.29%	14.92%
Net Operating Income	34,050	33,863	67,609	67,900
Net Operating Income: % of Total	12.33%	13.79%	12.50%	14.01%
New Jersey [Member]
Rental Revenue: Class A	15,811	16,526	32,090	33,005
Rental Revenue: Total	15,811	16,526	32,090	33,005
Rental Revenue: % of Total	3.70%	4.41%	3.83%	4.43%
Real Estate Operating Expenses: Class A	7,487	7,792	15,402	15,630
Real Estate Operating Expenses: Total	7,487	7,792	15,402	15,630
Real Estate Operating Expenses: % of Total	4.95%	6.02%	5.19%	6.02%
Net Operating Income	$ 8,324	$ 8,734	$ 16,688	$ 17,375
Net Operating Income: % of Total	3.01%	3.56%	3.09%	3.59%