Information by Business Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011 Year property segment	Sep. 30, 2010	Sep. 30, 2011 sqfeet Year property segment	Sep. 30, 2010	Dec. 31, 2010
Information by Business Segment
Number of primary office property segments	9		9
Segment financial information for real estate operations
Revenues from real estate operations	$ 125,129	$ 115,028	$ 368,376	$ 338,015
Property operating expenses	48,712	44,304	145,071	132,744
NOI from real estate operations	76,417	70,724	223,305	205,271
Segment assets	3,965,392		3,965,392		3,844,517
Operating Segment Total
Segment financial information for real estate operations
Revenues from real estate operations	125,129	115,028	368,376	338,015
Property operating expenses	48,712	44,304	145,071	132,744
NOI from real estate operations	76,417	70,724	223,305	205,271
Additions to properties, net	111,356	259,568	262,703	405,290
Segment assets	3,965,392	3,737,372	3,965,392	3,737,372
Baltimore/Washington Corridor
Segment financial information for real estate operations
Revenues from real estate operations	54,744	51,946	160,856	154,627
Property operating expenses	19,905	18,945	59,620	57,953
NOI from real estate operations	34,839	33,001	101,236	96,674
Additions to properties, net	22,208	19,097	65,551	67,313
Segment assets	1,414,086	1,382,234	1,414,086	1,382,234
Greater Baltimore
Segment financial information for real estate operations
Revenues from real estate operations	18,193	18,288	53,651	52,980
Property operating expenses	7,621	7,828	23,430	24,149
NOI from real estate operations	10,572	10,460	30,221	28,831
Additions to properties, net	2,706	14,578	20,390	29,737
Segment assets	551,942	584,429	551,942	584,429
Northern Virginia
Segment financial information for real estate operations
Revenues from real estate operations	18,640	18,949	55,359	55,780
Property operating expenses	7,364	7,195	22,409	21,214
NOI from real estate operations	11,276	11,754	32,950	34,566
Additions to properties, net	20,402	7,302	39,716	44,896
Segment assets	571,940	487,898	571,940	487,898
Colorado Springs
Segment financial information for real estate operations
Revenues from real estate operations	6,037	6,176	17,869	18,662
Property operating expenses	2,573	2,380	7,086	6,928
NOI from real estate operations	3,464	3,796	10,783	11,734
Additions to properties, net	1,293	1,028	2,712	2,541
Segment assets	261,255	266,228	261,255	266,228
Suburban Maryland
Segment financial information for real estate operations
Revenues from real estate operations	5,648	5,243	16,582	16,524
Property operating expenses	2,021	2,618	6,973	7,518
NOI from real estate operations	3,627	2,625	9,609	9,006
Additions to properties, net	4,372	1,373	7,935	3,454
Segment assets	176,336	175,181	176,336	175,181
San Antonio
Segment financial information for real estate operations
Revenues from real estate operations	7,701	5,609	22,453	13,775
Property operating expenses	3,897	2,697	10,974	6,426
NOI from real estate operations	3,804	2,912	11,479	7,349
Additions to properties, net	1,021	5,701	5,710	16,199
Segment assets	159,238	151,058	159,238	151,058
Washington, DC - Capitol Riverfront
Segment financial information for real estate operations
Revenues from real estate operations	4,507	135	13,349	135
Property operating expenses	1,808	50	5,092	50
NOI from real estate operations	2,699	85	8,257	85
Additions to properties, net	(262)	92,816	433	92,816
Segment assets	114,249	122,737	114,249	122,737
Greater Philadelphia
Segment financial information for real estate operations
Revenues from real estate operations	1,701	1,793	5,315	4,505
Property operating expenses	462	232	1,283	1,795
NOI from real estate operations	1,239	1,561	4,032	2,710
Additions to properties, net	4,786	2,187	10,819	18,518
Segment assets	133,077	121,695	133,077	121,695
St. Mary's and King George Counties
Segment financial information for real estate operations
Revenues from real estate operations	3,508	3,431	10,606	10,550
Property operating expenses	1,143	1,152	3,129	3,300
NOI from real estate operations	2,365	2,279	7,477	7,250
Additions to properties, net	2,795	3,445	9,939	3,988
Segment assets	104,732	96,813	104,732	96,813
Wholesale Data Center
Segment financial information for real estate operations
Revenues from real estate operations	1,283	162	3,769	162
Property operating expenses	1,054	251	2,591	251
NOI from real estate operations	229	(89)	1,178	(89)
Additions to properties, net	21,941	111,510	61,322	111,510
Segment assets	190,495	115,722	190,495	115,722
Other
Segment financial information for real estate operations
Revenues from real estate operations	3,167	3,296	8,567	10,315
Property operating expenses	864	956	2,484	3,160
NOI from real estate operations	2,303	2,340	6,083	7,155
Additions to properties, net	30,094	531	38,176	14,318
Segment assets	$ 288,042	$ 233,377	$ 288,042	$ 233,377

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Costs incurred and are directly related to generating revenues from leased and rented property or equipment for the reportable segment.

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Revenue from real estate operations for the reportable segment less property operating expenses for the reportable segment.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The cash outflow from the acquisition of a piece of land and anything permanently fixed to it, including buildings, structures on it and so forth, and payments to develop real estate assets by adding improvements on or to a parcel of land. Such improvements may include drainage, utilities, subdividing, access, buildings, and any combination of these elements. Including real estate intended to generate income for the owner; excluding real estate acquired for use by the owner.

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

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The aggregate revenue from real estate operations during the reporting period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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