Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended	3 Months Ended						12 Months Ended	3 Months Ended
	Mar. 31, 2013 item sqft	Mar. 31, 2012 sqft	Dec. 31, 2012 Office property	Mar. 31, 2013 CBD Office property sqft	Mar. 31, 2012 CBD Office sqft	Mar. 31, 2013 Suburban Office property sqft	Mar. 31, 2012 Suburban Office sqft	Mar. 31, 2013 Industrial and Other property sqft	Mar. 31, 2012 Industrial and Other sqft	Dec. 31, 2012 Industrial property	Mar. 31, 2013 Oahu, HI sqft	Mar. 31, 2012 Oahu, HI sqft	Mar. 31, 2013 Oahu, HI Industrial and Other sqft	Mar. 31, 2012 Oahu, HI Industrial and Other sqft	Mar. 31, 2013 Metro Chicago, IL sqft	Mar. 31, 2012 Metro Chicago, IL sqft	Mar. 31, 2013 Metro Chicago, IL CBD Office sqft	Mar. 31, 2012 Metro Chicago, IL CBD Office sqft	Mar. 31, 2013 Metro Chicago, IL Suburban Office sqft	Mar. 31, 2012 Metro Chicago, IL Suburban Office sqft	Mar. 31, 2013 Metro Chicago, IL Industrial and Other sqft	Mar. 31, 2012 Metro Chicago, IL Industrial and Other sqft	Mar. 31, 2013 Metro Philadelphia, PA sqft	Mar. 31, 2012 Metro Philadelphia, PA sqft	Mar. 31, 2013 Metro Philadelphia, PA CBD Office sqft	Mar. 31, 2012 Metro Philadelphia, PA CBD Office sqft	Mar. 31, 2013 Metro Philadelphia, PA Suburban Office sqft	Mar. 31, 2012 Metro Philadelphia, PA Suburban Office sqft	Mar. 31, 2013 Other Markets sqft	Mar. 31, 2012 Other Markets sqft	Mar. 31, 2013 Other Markets CBD Office sqft	Mar. 31, 2012 Other Markets CBD Office sqft	Mar. 31, 2013 Other Markets Suburban Office sqft	Mar. 31, 2012 Other Markets Suburban Office sqft	Mar. 31, 2013 Other Markets Industrial and Other sqft	Mar. 31, 2012 Other Markets Industrial and Other sqft
Segment information
Number of reportable segments	3
Properties owned				54		246		145
Number of properties reclassified to discontinued operations			40			3				57
Number of real estate properties reclassified from one geographical segment to anonther geographical segment				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%
Property square feet	72,690,000	67,152,000		22,048,000	20,317,000	20,345,000	18,122,000	30,297,000	28,713,000		17,894,000	17,876,000	17,894,000	17,876,000	4,868,000	4,859,000	3,601,000	3,591,000	1,164,000	1,164,000	103,000	104,000	4,852,000	4,846,000	4,597,000	4,590,000	255,000	256,000	45,076,000	39,571,000	13,850,000	12,136,000	18,926,000	16,702,000	12,300,000	10,733,000
Property rental income	$ 275,048	$ 243,378		$ 142,751	$ 128,139	$ 87,133	$ 75,143	$ 45,164	$ 40,096		$ 21,211	$ 19,895	$ 21,211	$ 19,895	$ 31,851	$ 30,549	$ 25,149	$ 24,575	$ 6,591	$ 5,863	$ 111	$ 111	$ 30,006	$ 30,144	$ 29,131	$ 29,300	$ 875	$ 844	$ 191,980	$ 162,790	$ 88,471	$ 74,264	$ 79,667	$ 68,436	$ 23,842	$ 20,090
Property NOI	$ 165,389	$ 146,142		$ 78,693	$ 71,975	$ 51,904	$ 44,139	$ 34,792	$ 30,028		$ 16,737	$ 15,514	$ 16,737	$ 15,514	$ 16,041	$ 15,445	$ 12,269	$ 12,378	$ 3,670	$ 2,963	$ 102	$ 104	$ 16,001	$ 15,787	$ 15,796	$ 15,584	$ 205	$ 203	$ 116,610	$ 99,396	$ 50,628	$ 44,013	$ 48,029	$ 40,973	$ 17,953	$ 14,410