Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended				6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended			6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 sqft	Jun. 30, 2010 sqft	Jun. 30, 2011 Metro Philadelphia, PA	Jun. 30, 2010 Metro Philadelphia, PA	Jun. 30, 2011 Metro Philadelphia, PA sqft	Jun. 30, 2010 Metro Philadelphia, PA sqft	Jun. 30, 2011 Metro Philadelphia, PA Suburban Office	Jun. 30, 2010 Metro Philadelphia, PA Suburban Office	Jun. 30, 2011 Metro Philadelphia, PA Suburban Office sqft	Jun. 30, 2010 Metro Philadelphia, PA Suburban Office sqft	Jun. 30, 2011 Metro Philadelphia, PA CBD Office	Jun. 30, 2010 Metro Philadelphia, PA CBD Office	Jun. 30, 2011 Metro Philadelphia, PA CBD Office sqft	Jun. 30, 2010 Metro Philadelphia, PA CBD Office sqft	Jun. 30, 2011 Industrial and Other Oahu, HI	Jun. 30, 2010 Industrial and Other Oahu, HI	Jun. 30, 2011 Industrial and Other Oahu, HI sqft	Jun. 30, 2010 Industrial and Other Oahu, HI sqft	Jun. 30, 2011 Oahu, HI	Jun. 30, 2010 Oahu, HI	Jun. 30, 2011 Oahu, HI sqft	Jun. 30, 2010 Oahu, HI sqft	Jun. 30, 2011 Industrial and Other Metro Denver, CO	Jun. 30, 2010 Industrial and Other Metro Denver, CO	Jun. 30, 2011 Industrial and Other Metro Denver, CO sqft	Jun. 30, 2010 Industrial and Other Metro Denver, CO sqft	Jun. 30, 2011 Metro Denver, CO	Jun. 30, 2010 Metro Denver, CO	Jun. 30, 2011 Metro Denver, CO sqft	Jun. 30, 2010 Metro Denver, CO sqft	Jun. 30, 2011 Metro Denver, CO Suburban Office	Jun. 30, 2010 Metro Denver, CO Suburban Office	Jun. 30, 2011 Metro Denver, CO Suburban Office sqft	Jun. 30, 2010 Metro Denver, CO Suburban Office sqft	Jun. 30, 2011 Metro Denver, CO CBD Office	Jun. 30, 2010 Metro Denver, CO CBD Office	Jun. 30, 2011 Metro Denver, CO CBD Office sqft	Jun. 30, 2010 Metro Denver, CO CBD Office sqft	Jun. 30, 2011 Industrial and Other Metro Chicago, IL	Jun. 30, 2010 Industrial and Other Metro Chicago, IL	Jun. 30, 2011 Industrial and Other Metro Chicago, IL sqft	Jun. 30, 2010 Industrial and Other Metro Chicago, IL sqft	Jun. 30, 2011 Metro Chicago, IL	Jun. 30, 2010 Metro Chicago, IL	Jun. 30, 2011 Metro Chicago, IL sqft	Jun. 30, 2010 Metro Chicago, IL sqft	Jun. 30, 2011 Metro Chicago, IL Suburban Office	Jun. 30, 2010 Metro Chicago, IL Suburban Office	Jun. 30, 2011 Metro Chicago, IL Suburban Office sqft	Jun. 30, 2010 Metro Chicago, IL Suburban Office sqft	Jun. 30, 2011 Metro Chicago, IL CBD Office	Jun. 30, 2011 Metro Chicago, IL CBD Office sqft	Jun. 30, 2011 Industrial and Other Australia	Jun. 30, 2011 Industrial and Other Australia sqft	Jun. 30, 2011 Australia	Jun. 30, 2011 Australia sqft	Jun. 30, 2011 Australia CBD Office	Jun. 30, 2011 Australia CBD Office sqft	Jun. 30, 2011 Metro Washington, DC	Jun. 30, 2010 Metro Washington, DC	Jun. 30, 2011 Metro Washington, DC sqft	Jun. 30, 2010 Metro Washington, DC sqft	Jun. 30, 2011 Metro Washington, DC Suburban Office	Jun. 30, 2010 Metro Washington, DC Suburban Office	Jun. 30, 2011 Metro Washington, DC Suburban Office sqft	Jun. 30, 2010 Metro Washington, DC Suburban Office sqft	Jun. 30, 2011 Metro Washington, DC CBD Office	Jun. 30, 2010 Metro Washington, DC CBD Office	Jun. 30, 2011 Metro Washington, DC CBD Office sqft	Jun. 30, 2010 Metro Washington, DC CBD Office sqft	Jun. 30, 2011 Industrial and Other Other markets	Jun. 30, 2010 Industrial and Other Other markets	Jun. 30, 2011 Industrial and Other Other markets sqft	Jun. 30, 2010 Industrial and Other Other markets sqft	Jun. 30, 2011 Other markets	Jun. 30, 2010 Other markets	Jun. 30, 2011 Other markets sqft	Jun. 30, 2010 Other markets sqft	Jun. 30, 2011 Other markets Suburban Office	Jun. 30, 2010 Other markets Suburban Office	Jun. 30, 2011 Other markets Suburban Office sqft	Jun. 30, 2010 Other markets Suburban Office sqft	Jun. 30, 2011 Other markets CBD Office	Jun. 30, 2010 Other markets CBD Office	Jun. 30, 2011 Other markets CBD Office sqft	Jun. 30, 2010 Other markets CBD Office sqft	Dec. 31, 2010 Office	Sep. 30, 2010 Office	Jun. 30, 2011 Suburban Office	Jun. 30, 2010 Suburban Office	Jun. 30, 2011 Suburban Office sqft	Jun. 30, 2010 Suburban Office sqft	Jun. 30, 2011 CBD Office	Jun. 30, 2010 CBD Office	Jun. 30, 2011 CBD Office sqft	Jun. 30, 2010 CBD Office sqft	Jun. 30, 2011 Industrial and Other	Dec. 31, 2010 Industrial and Other	Jun. 30, 2010 Industrial and Other	Jun. 30, 2011 Industrial and Other sqft	Jun. 30, 2010 Industrial and Other sqft
Segment information
Properties owned																																																																																											279		279		40		40		180			180
Factors used to define individual geographic segments			We define these individual geographic segments as those which currently, or during either of the last two quarters, represent or generate 5% or more of our total square feet, annualized revenues or property net operating income, or NOI, which we define as rental income less operating expenses
Geographic segments, number of immediately preceding quarters			2
Geographic segments, minimum percentage of total square feet, annualized revenues or property net operating income, or net operating income defined as rental income less operating expenses (as a percent)			5.00%
Number of properties reclassified to discontinued operations																																																																																									30	1										25
Property square feet (in square feet)			67,467,000	61,306,000			5,211,000	5,204,000			620,000	619,000			4,591,000	4,585,000			17,914,000	17,914,000			17,914,000	17,914,000			553,000	553,000			2,014,000	2,014,000			789,000	789,000			672,000	672,000			104,000	104,000			2,537,000	834,000			1,361,000	730,000		1,072,000		1,440,000		1,754,000		314,000			1,644,000	1,869,000			1,216,000	1,287,000			428,000	582,000			10,598,000	10,420,000			36,393,000	33,471,000			18,494,000	16,662,000			7,301,000	6,389,000					22,480,000	20,087,000			14,378,000	12,228,000				30,609,000	28,991,000
Property rental income	$ 222,181	$ 195,817	$ 436,697	$ 390,896	$ 30,410	$ 30,255	$ 60,505	$ 61,127	$ 1,919	$ 2,685	$ 3,895	$ 4,882	$ 28,491	$ 27,570	$ 56,610	$ 56,245	$ 18,094	$ 18,566	$ 36,682	$ 36,343	$ 18,094	$ 18,566	$ 36,682	$ 36,343	$ 2,166	$ 2,035	$ 4,350	$ 4,109	$ 11,304	$ 10,817	$ 22,341	$ 19,807	$ 3,459	$ 3,410	$ 6,949	$ 5,221	$ 5,679	$ 5,372	$ 11,042	$ 10,477	$ 111	$ 126	$ 236	$ 251	$ 12,302	$ 4,180	$ 21,331	$ 8,796	$ 8,112	$ 4,054	$ 17,016	$ 8,545	$ 4,079	$ 4,079	$ 3,195	$ 6,092	$ 8,952	$ 17,020	$ 5,757	$ 10,928	$ 9,668	$ 12,631	$ 18,980	$ 25,271	$ 6,315	$ 7,351	$ 12,203	$ 14,900	$ 3,353	$ 5,280	$ 6,777	$ 10,371	$ 15,903	$ 16,729	$ 31,371	$ 32,876	$ 131,451	$ 119,368	$ 259,838	$ 239,552	$ 73,793	$ 65,238	$ 147,522	$ 130,706	$ 41,755	$ 37,401	$ 80,945	$ 75,970			$ 93,598	$ 82,738	$ 187,585	$ 164,254	$ 89,114	$ 75,623	$ 170,381	$ 153,063	$ 39,469		$ 37,456	$ 78,731	$ 73,579
Property net operating income	$ 132,020	$ 114,465	$ 256,129	$ 228,074	$ 15,179	$ 15,186	$ 29,852	$ 30,702	$ 371	$ 1,050	$ 787	$ 1,747	$ 14,808	$ 14,136	$ 29,065	$ 28,955	$ 13,802	$ 13,750	$ 27,215	$ 27,027	$ 13,802	$ 13,750	$ 27,215	$ 27,027	$ 1,272	$ 1,185	$ 2,480	$ 2,391	$ 7,877	$ 7,225	$ 15,353	$ 13,134	$ 2,786	$ 2,551	$ 5,554	$ 3,626	$ 3,819	$ 3,489	$ 7,319	$ 7,117	$ 103	$ 102	$ 213	$ 201	$ 7,141	$ 2,751	$ 12,520	$ 5,892	$ 4,857	$ 2,649	$ 10,126	$ 5,691	$ 2,181	$ 2,181	$ 2,424	$ 4,424	$ 7,039	$ 13,427	$ 4,615	$ 9,003	$ 6,196	$ 7,804	$ 12,348	$ 15,712	$ 3,878	$ 4,230	$ 7,421	$ 8,656	$ 2,318	$ 3,574	$ 4,927	$ 7,056	$ 10,710	$ 12,655	$ 21,076	$ 24,270	$ 74,786	$ 67,749	$ 145,414	$ 135,607	$ 43,228	$ 36,820	$ 83,612	$ 73,868	$ 20,848	$ 18,274	$ 40,726	$ 37,469			$ 55,120	$ 47,300	$ 107,500	$ 93,588	$ 48,589	$ 39,473	$ 93,221	$ 80,597	$ 28,311		$ 27,692	$ 55,408	$ 53,889