Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012 sqft	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011 sqft	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 item sqft	Dec. 31, 2011 sqft	Dec. 31, 2010 sqft
Segment information
Number of reportable segments									3
Geographic segments, immediately preceding period									6 months
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%
Total real estate properties, at cost, net	$ 6,821,803				$ 6,310,062				$ 6,821,803	$ 6,310,062
Property square feet	71,901,000				65,311,000				71,901,000	65,311,000	58,736,000
Property rental income	264,692	255,225	249,797	243,378	233,528	232,282	208,883	199,539	1,013,092	874,232	720,747
Property net operating income									593,411	510,768	417,846
Office
Segment information
Properties owned	295								295
Number of properties reclassified to discontinued operations									40
CBD Office
Segment information
Properties owned	51								51
Property square feet	21,253,000				17,654,000				21,253,000	17,654,000	12,413,000
Property rental income									509,871	390,191	305,488
Property net operating income									270,366	210,910	158,553
Suburban Office
Segment information
Properties owned	244								244
Property square feet	20,351,000				18,901,000				20,351,000	18,901,000	17,718,000
Property rental income									340,087	329,279	275,548
Property net operating income									201,821	187,882	156,440
Industrial and Other
Segment information
Properties owned	145								145
Property square feet	30,297,000				28,756,000				30,297,000	28,756,000	28,605,000
Property rental income									163,134	154,762	139,711
Property net operating income									121,224	111,976	102,853
Industrial
Segment information
Number of properties reclassified to discontinued operations									57
Metro Philadelphia, PA
Segment information
Property square feet	4,851,000				4,846,000				4,851,000	4,846,000	4,847,000
Property rental income									121,675	118,514	116,761
Property net operating income									64,211	59,627	58,905
Metro Philadelphia, PA | CBD Office
Segment information
Property square feet	4,596,000				4,591,000				4,596,000	4,591,000	4,592,000
Property rental income									118,046	114,640	112,676
Property net operating income									63,239	58,917	58,272
Metro Philadelphia, PA | Suburban Office
Segment information
Property square feet	255,000				255,000				255,000	255,000	255,000
Property rental income									3,629	3,874	4,085
Property net operating income									972	710	633
Oahu, HI
Segment information
Property square feet	17,894,000				17,896,000				17,894,000	17,896,000	17,914,000
Property rental income									75,968	73,413	74,150
Property net operating income									58,852	55,039	55,702
Oahu, HI | Industrial and Other
Segment information
Property square feet	17,894,000				17,896,000				17,894,000	17,896,000	17,914,000
Property rental income									75,968	73,413	74,150
Property net operating income									58,852	55,039	55,702
Metro Chicago, IL
Segment information
Property square feet	4,868,000				3,849,000				4,868,000	3,849,000	1,268,000
Property rental income									130,551	66,562	16,455
Property net operating income									65,178	37,311	11,244
Metro Chicago, IL | CBD Office
Segment information
Property square feet	3,600,000				2,581,000				3,600,000	2,581,000
Property rental income									104,713	38,256
Property net operating income									51,616	21,170
Metro Chicago, IL | Suburban Office
Segment information
Property square feet	1,164,000				1,164,000				1,164,000	1,164,000	1,164,000
Property rental income									25,393	27,837	15,966
Property net operating income									13,100	15,710	10,840
Metro Chicago, IL | Industrial and Other
Segment information
Property square feet	104,000				104,000				104,000	104,000	104,000
Property rental income									445	469	489
Property net operating income									462	431	404
Other Markets
Segment information
Property square feet	44,288,000				38,720,000				44,288,000	38,720,000	34,707,000
Property rental income									684,898	615,743	513,381
Property net operating income									405,170	358,791	291,995
Other Markets | CBD Office
Segment information
Property square feet	13,057,000				10,482,000				13,057,000	10,482,000	7,821,000
Property rental income									287,112	237,295	192,812
Property net operating income									155,511	130,823	100,281
Other Markets | Suburban Office
Segment information
Property square feet	18,932,000				17,482,000				18,932,000	17,482,000	16,299,000
Property rental income									311,065	297,568	255,497
Property net operating income									187,749	171,462	144,967
Other Markets | Industrial and Other
Segment information
Property square feet	12,299,000				10,756,000				12,299,000	10,756,000	10,587,000
Property rental income									86,721	80,880	65,072
Property net operating income									61,910	56,506	46,747
Australia | CBD Office
Segment information
Total real estate properties, at cost, net	168,220				169,399				168,220	169,399
Australia | Industrial
Segment information
Total real estate properties, at cost, net	$ 104,010				$ 100,588				$ 104,010	$ 100,588