Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended		1 Months Ended																		12 Months Ended	1 Months Ended		9 Months Ended
	Sep. 30, 2010	Sep. 30, 2011 sqft property quarter	Sep. 30, 2010 sqft	Jan. 31, 2011 Boca Raton, FL Acquisition Office property 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 property sqft	Aug. 31, 2011 Chicago, IL Acquisition Office sqft property	May 31, 2011 Chicago, IL Acquisition Office sqft	Aug. 31, 2011 Chicago, IL Agreement to acquire Office sqft	Jun. 30, 2011 Stafford, VA Acquisition Office property sqft	Jun. 30, 2011 Folsom, CA Acquisition Office sqft property	Jul. 31, 2011 Birmingham, AL Acquisition Office sqft	Aug. 31, 2011 New Orleans, LA Acquisition Office sqft	Oct. 31, 2011 Hartford, CT Agreement to acquire Office sqft	Feb. 28, 2011 Adairsville, GA Sale Industrial & Other sqft	Oct. 31, 2011 Dearborn, MI SNH Agreement to sell Industrial property sqft	Sep. 30, 2011 SNH property sqft state	Sep. 30, 2011 SNH Sale sqft	Jan. 31, 2011 SNH Sale property sqft	Dec. 31, 2010 SNH Sale property sqft	Sep. 30, 2011 SNH Agreement to sell	Nov. 30, 2010 SNH Agreement to sell property sqft	Sep. 30, 2011 Real estate properties held for sale Office sqft property	Sep. 30, 2011 Real estate properties held for sale Industrial & Other sqft property	Sep. 30, 2011 Acquisition property sqft	Sep. 30, 2011 Sale property sqft	Sep. 30, 2011 Agreement to acquire sqft property	Sep. 30, 2011 Agreement to sell sqft property	Sep. 30, 2011 Industrial & Other sqft property	Sep. 30, 2010 Industrial & Other sqft
Real estate properties information
Number of properties acquired or agreed to be acquired		23		3				4	2			4	4																2
Number of properties acquired and continue to own																											22
Property square feet		69,399,000	59,430,000	639,830	115,028	98,048	119,089	1,063,364	1,510,707	1,070,388	1,006,574	149,023	269,254	514,893	1,256,991	884,669	101,400	570,000	1,310,000	1,310,000	737,000	2,066,000		2,803,000	1,054,000	1,835,000	6,806,615	2,148,000	1,891,243	570,000	30,485,000	28,884,000
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs		$ 1,144,852		$ 171,000	$ 12,025	$ 10,000	$ 20,600	$ 136,500	$ 390,000	$ 162,202	$ 150,600	$ 25,725	$ 46,300		$ 102,000	$ 99,000				$ 167,000							$ 1,132,827		$ 249,600
Assumption of mortgage debt		321,235							265,000		148,000	14,960	41,275														321,235		148,000
Number of properties sold or agreed to be sold																		16	13		6	21		27				20		16
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs														68,500			2,275	6,500			95,870	374,130	167,000	470,000				265,145		6,500
Number of states in which properties are located																			8
Funded real estate improvements		65,176
Real estate purchase price allocation, land				15,900	1,180	1,410	3,650	30,985	34,980	34,300		4,150	4,370	1,740	9,100
Real estate purchase price allocation, buildings and improvements				129,790	8,886	7,322	13,726	55,733	310,574	110,245		21,795	41,748	49,565	78,540
Real estate purchase price allocation, (discount) premium on mortgage debt									13,671			934	3,285
Real estate purchase price allocation, acquired real estate leases				25,310	2,072	1,711	3,954	15,706	62,016	24,399		815	3,729	17,552	17,743
Real estate purchase price allocation, direct financing lease								38,635
Real estate purchase price allocation, assumed real estate lease obligations					113	443	730	500	3,899	6,742		101	262	357	3,383
Real estate purchase price allocation, notes payable								4,059
Gain (loss) on sale of properties	$ 22,832		$ 34,336														$ (94)			$ 7,001	$ 34,666	$ 133,272
Properties owned																									7	20					179