Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2011 property	Dec. 31, 2010 property	Sep. 30, 2010	Sep. 30, 2011 sqft property	Sep. 30, 2010 sqft
Segment information
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				5.00%
Property square feet				69,399,000	59,430,000
Property rental income	$ 238,790		$ 193,059	$ 662,596	$ 572,205
Property net operating income	137,878		110,036	386,836	331,925
Metro Philadelphia, PA
Segment information
Property square feet				5,053,000	5,054,000
Property rental income	30,547		29,792	89,736	89,687
Property net operating income	15,103		14,786	44,303	45,031
Metro Philadelphia, PA | CBD Office
Segment information
Property square feet				4,591,000	4,592,000
Property rental income	29,169		27,920	85,779	84,165
Property net operating income	14,854		14,112	43,919	43,066
Metro Philadelphia, PA | Suburban Office
Segment information
Property square feet				462,000	462,000
Property rental income	1,378		1,872	3,957	5,522
Property net operating income	249		674	384	1,965
Industrial & Other | Oahu, HI
Segment information
Property square feet				17,896,000	17,914,000
Property rental income	18,190		18,114	54,872	54,457
Property net operating income	13,588		13,542	40,803	40,569
Oahu, HI
Segment information
Property square feet				17,896,000	17,914,000
Property rental income	18,190		18,114	54,872	54,457
Property net operating income	13,588		13,542	40,803	40,569
Industrial & Other | Metro Chicago, IL
Segment information
Property square feet				104,000	104,000
Property rental income	111		116	347	366
Property net operating income	104		101	317	302
Metro Chicago, IL
Segment information
Property square feet				3,850,000	636,000
Property rental income	21,166		3,557	40,408	10,017
Property net operating income	11,831		2,530	23,329	7,061
Metro Chicago, IL | CBD Office
Segment information
Property square feet				2,582,000
Property rental income	14,272			18,351
Property net operating income	8,044			10,225
Metro Chicago, IL | Suburban Office
Segment information
Property square feet				1,164,000	532,000
Property rental income	6,783		3,441	21,710	9,651
Property net operating income	3,683		2,429	12,787	6,759
Metro Washington, DC
Segment information
Property square feet				1,644,000	1,495,000
Property rental income	13,498		11,755	32,478	37,026
Property net operating income	9,899		7,119	22,247	22,831
Metro Washington, DC | CBD Office
Segment information
Property square feet				428,000	428,000
Property rental income	4,726		4,284	11,503	14,655
Property net operating income	3,942		2,747	8,869	9,803
Metro Washington, DC | Suburban Office
Segment information
Property square feet				1,216,000	1,067,000
Property rental income	8,772		7,471	20,975	22,371
Property net operating income	5,957		4,372	13,378	13,028
Industrial & Other | Metro Denver, CO
Segment information
Property square feet				553,000	553,000
Property rental income	2,324		2,124	6,674	6,233
Property net operating income	1,191		1,201	3,671	3,592
Metro Denver, CO
Segment information
Property square feet				2,014,000	2,013,000
Property rental income	11,327		11,140	33,668	30,947
Property net operating income	7,412		7,383	22,766	20,517
Metro Denver, CO | CBD Office
Segment information
Property square feet				672,000	672,000
Property rental income	4,998		5,238	16,040	15,715
Property net operating income	3,136		3,409	10,455	10,526
Metro Denver, CO | Suburban Office
Segment information
Property square feet				789,000	788,000
Property rental income	4,005		3,778	10,954	8,999
Property net operating income	3,085		2,773	8,640	6,399
Industrial & Other | Australia
Segment information
Property square feet				1,442,000
Property rental income	3,230			9,322
Property net operating income	2,438			6,862
Australia
Segment information
Property square feet				1,756,000
Property rental income	8,495			25,515
Property net operating income	6,734			20,161
Australia | CBD Office
Segment information
Property square feet				314,000
Property rental income	5,265			16,193
Property net operating income	4,296			13,299
Industrial & Other | Other markets
Segment information
Property square feet				10,490,000	10,313,000
Property rental income	16,668		16,760	47,809	49,461
Property net operating income	11,031		12,190	31,966	36,383
Other markets
Segment information
Property square feet				37,186,000	32,318,000
Property rental income	135,567		118,701	385,919	350,071
Property net operating income	73,311		64,676	213,227	195,916
Other markets | CBD Office
Segment information
Property square feet				9,067,000	6,423,000
Property rental income	46,569		38,210	127,514	114,180
Property net operating income	22,613		17,727	63,339	55,197
Other markets | Suburban Office
Segment information
Property square feet				17,629,000	15,582,000
Property rental income	72,330		63,731	210,596	186,430
Property net operating income	39,667		34,759	117,922	104,336
Office
Segment information
Number of properties reclassified to discontinued operations		30		12
CBD Office
Segment information
Properties owned	44			44
Property square feet				17,654,000	12,115,000
Property rental income	104,999		75,652	275,380	228,715
Property net operating income	56,885		37,995	150,106	118,592
Suburban Office
Segment information
Properties owned	266			266
Property square feet				21,260,000	18,431,000
Property rental income	93,268		80,293	268,192	232,973
Property net operating income	52,641		45,007	153,111	132,487
Industrial & Other
Segment information
Properties owned	179			179
Number of properties reclassified to discontinued operations		25		1
Property square feet				30,485,000	28,884,000
Property rental income	40,523		37,114	119,024	110,517
Property net operating income	$ 28,352		$ 27,034	$ 83,619	$ 80,846