Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended		1 Months Ended														12 Months Ended	1 Months Ended		6 Months Ended
	Jun. 30, 2010	Jun. 30, 2011 sqft	Jun. 30, 2010 sqft	Jan. 31, 2011 Boca Raton, FL Acquisition Office sqft	Jan. 31, 2011 Columbia, SC Acquisition Office sqft	Jan. 31, 2011 Chelmsford, MA Acquisition Office sqft	Feb. 28, 2011 Montvale, NJ Acquisition Office sqft	Mar. 31, 2011 Phoenix, AZ Acquisition Office sqft	May 31, 2011 Chicago, IL Acquisition Office sqft	May 31, 2011 Chicago, IL Agreement to acquire Office sqft	Jun. 30, 2011 Stafford, VA Acquisition Office sqft	Jun. 30, 2011 Folsom, CA Acquisition Office sqft	Jul. 31, 2011 Birmingham, AL Acquisition Office sqft	Jul. 31, 2011 New Orleans, LA Agreement to acquire Office sqft	Jul. 31, 2011 Portland, OR Agreement to acquire Office sqft	Feb. 28, 2011 Adairsville, GA Sale Industrial and Other sqft	Jan. 31, 2011 SNH Sale sqft	Dec. 31, 2010 SNH Sale sqft	Nov. 30, 2010 SNH Agreement to sell sqft	May 31, 2008 SNH Agreement to sell sqft	Jun. 30, 2011 Real estate properties held for sale Office sqft	Jun. 30, 2011 Real estate properties held for sale Industrial and Other sqft	Jun. 30, 2011 Acquisition sqft	Jun. 30, 2011 Sale sqft	Jun. 30, 2011 Agreement to acquire sqft	Jun. 30, 2011 Industrial and Other sqft	Jun. 30, 2010 Industrial and Other sqft
Real estate properties information
Number of properties acquired or agreed to be acquired				3				4		2	4	4											20		4
Property square feet (in square feet)		67,467,000	61,306,000	639,830	115,028	98,048	119,089	1,063,364	1,070,388	1,571,386	149,023	269,254	514,893	1,256,991	106,658	101,400	737,000	2,066,000	2,803,000	2,161,000	1,054,000	1,835,000	4,039,000	838,000	2,935,000	30,609,000	28,991,000
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs				$ 171,000	$ 12,025	$ 10,000	$ 20,600	$ 136,500	$ 162,202	$ 390,000	$ 25,725	$ 46,300	$ 68,500	$ 107,000	$ 34,100								$ 652,852		$ 531,100
Assumption of mortgage debt		56,235								265,000	14,960	41,275											56,235		265,000
Number of properties sold or agreed to be sold																	6	21	27					7
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs																2,275	95,870	374,130	470,000	562,000				98,145
Funded real estate improvements		39,915
Real estate purchase price allocation, land				15,900	1,180	1,410	3,650	30,985	34,300		4,150	4,370
Real estate purchase price allocation, buildings and improvements				129,790	8,886	7,322	13,726	55,733	110,245		21,795	41,748
Real estate purchase price allocation, discount (premium) on mortgage debt											934	3,285
Real estate purchase price allocation, acquired real estate leases				25,310	2,072	1,711	3,954	15,706	24,399		815	3,729
Real estate purchase price allocation, direct financing lease								38,635
Real estate purchase price allocation, assumed real estate lease obligations					113	443	730	500	6,742		101	262
Real estate purchase price allocation, notes payable								4,059
Gain (loss) on sale of properties	$ 11,504		$ 11,504													$ (94)	$ 34,666	$ 133,272
Properties owned																					7	20				180