Segment Information (Segment Reporting by Geographic Area and Property Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Rental Revenue: Class A	$ 421,629	$ 360,416	$ 1,220,842	$ 1,067,074
Rental Revenue: Office/Technical	12,106	11,539	35,265	34,691
Rental Revenue: Residential	2,453		3,029
Rental Revenue: Hotel	8,045	8,016	22,897	22,290
Rental Revenue: Total	444,233	379,971	1,282,033	1,124,055
Rental Revenue: % of Total	100.00%	100.00%	100.00%	100.00%
Real Estate Operating Expenses: Class A	148,958	124,631	426,056	366,383
Real Estate Operating Expenses: Office/Technical	3,609	3,410	10,634	9,927
Real Estate Operating Expenses: Residential	2,418		3,141
Real Estate Operating Expenses: Hotel	6,032	6,194	18,052	17,551
Real Estate Operating Expenses: Total	161,017	134,235	457,883	393,861
Real Estate Operating Expenses: % of Total	100.00%	100.00%	100.00%	100.00%
Net Operating Income	164,510	151,082	456,665	461,623
Net Operating Income: % of Total	100.00%	100.00%	100.00%	100.00%
Segment, Continuing Operations [Member]
Segment Reporting Information [Line Items]
Net Operating Income	283,216	245,736	824,150	730,194
Greater Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A	141,680	94,016	405,234	273,702
Rental Revenue: Office/Technical	7,871	7,627	23,088	22,808
Rental Revenue: Residential	284		284
Rental Revenue: Hotel	8,045	8,016	22,897	22,290
Rental Revenue: Total	157,880	109,659	451,503	318,800
Rental Revenue: % of Total	35.54%	28.86%	35.22%	28.36%
Real Estate Operating Expenses: Class A	55,614	36,082	154,580	103,481
Real Estate Operating Expenses: Office/Technical	2,501	2,417	7,392	6,790
Real Estate Operating Expenses: Residential	270		270
Real Estate Operating Expenses: Hotel	6,032	6,194	18,052	17,551
Real Estate Operating Expenses: Total	64,417	44,693	180,294	127,822
Real Estate Operating Expenses: % of Total	40.01%	33.29%	39.38%	32.46%
Net Operating Income	93,463	64,966	271,209	190,978
Net Operating Income: % of Total	33.00%	26.44%	32.91%	26.15%
Greater Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A	95,192	84,708	265,286	251,126
Rental Revenue: Office/Technical	4,235	3,912	12,177	11,883
Rental Revenue: Residential	2,169		2,745
Rental Revenue: Total	101,596	88,620	280,208	263,009
Rental Revenue: % of Total	22.87%	23.32%	21.86%	23.40%
Real Estate Operating Expenses: Class A	26,036	24,243	74,221	69,968
Real Estate Operating Expenses: Office/Technical	1,108	993	3,242	3,137
Real Estate Operating Expenses: Residential	2,148		2,871
Real Estate Operating Expenses: Total	29,292	25,236	80,334	73,105
Real Estate Operating Expenses: % of Total	18.19%	18.80%	17.54%	18.56%
Net Operating Income	72,304	63,384	199,874	189,904
Net Operating Income: % of Total	25.53%	25.79%	24.25%	26.01%
Midtown Manhattan [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A	115,284	111,383	341,698	332,301
Rental Revenue: Total	115,284	111,383	341,698	332,301
Rental Revenue: % of Total	25.95%	29.32%	26.65%	29.56%
Real Estate Operating Expenses: Class A	39,331	35,908	114,423	110,177
Real Estate Operating Expenses: Total	39,331	35,908	114,423	110,177
Real Estate Operating Expenses: % of Total	24.43%	26.75%	24.99%	27.97%
Net Operating Income	75,953	75,475	227,275	222,124
Net Operating Income: % of Total	26.82%	30.71%	27.58%	30.42%
Greater San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A	53,978	54,268	161,039	160,899
Rental Revenue: Total	53,978	54,268	161,039	160,899
Rental Revenue: % of Total	12.15%	14.28%	12.56%	14.32%
Real Estate Operating Expenses: Class A	20,788	20,317	60,241	59,047
Real Estate Operating Expenses: Total	20,788	20,317	60,241	59,047
Real Estate Operating Expenses: % of Total	12.91%	15.14%	13.16%	14.99%
Net Operating Income	33,190	33,951	100,798	101,852
Net Operating Income: % of Total	11.72%	13.82%	12.23%	13.95%
New Jersey [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A	15,495	16,041	47,585	49,046
Rental Revenue: Total	15,495	16,041	47,585	49,046
Rental Revenue: % of Total	3.49%	4.22%	3.71%	4.36%
Real Estate Operating Expenses: Class A	7,189	8,081	22,591	23,710
Real Estate Operating Expenses: Total	7,189	8,081	22,591	23,710
Real Estate Operating Expenses: % of Total	4.46%	6.02%	4.93%	6.02%
Net Operating Income	$ 8,306	$ 7,960	$ 24,994	$ 25,336
Net Operating Income: % of Total	2.93%	3.24%	3.03%	3.47%

Percentage Of Total Net Operating Income

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Real Estate Operating Expenses Class A Office

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Real Estate Operating Expenses Residential.

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Real Estate Operating Expenses Total

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Rental Revenue Class A Office

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The aggregate direct costs incurred in operating leased, owned and other hotels.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 03

-Paragraph 2

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Topic 225

-SubTopic 10

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-Paragraph 2

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Paragraph 27

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