Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		3 Months Ended	9 Months Ended		3 Months Ended		1 Months Ended			9 Months Ended				0 Months Ended		1 Months Ended
	Jun. 30, 2012	Apr. 30, 2012	Sep. 30, 2012 sqft	Sep. 30, 2012 sqft	Sep. 30, 2011 sqft	Dec. 31, 2011 Office property	Dec. 31, 2011 Industrial property	Sep. 30, 2012 Acquisition sqft	May 31, 2012 Acquisition	Jan. 31, 2012 Acquisition	Sep. 30, 2012 Acquisition property sqft	Sep. 30, 2012 Acquisition CWH and its wholly owned subsidiaries, including SIR sqft	Sep. 30, 2012 Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	Sep. 30, 2012 Acquisition SIR sqft	Nov. 06, 2012 Pending acquisitions SIR property sqft	Feb. 29, 2012 Sale SIR sqft	Mar. 31, 2011 Phoenix, AZ Acquisition property sqft	Jan. 31, 2012 Chicago, IL Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	Mar. 31, 2012 Hartford, CT Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	May 31, 2012 Austin, TX Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	Jun. 30, 2012 Provo, UT Acquisition SIR sqft	Jun. 30, 2012 Englewood, CO Acquisition SIR sqft	Sep. 30, 2012 Columbia, SC Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	Oct. 31, 2012 Indianapolis, IN Acquisition Office property sqft	Apr. 30, 2012 Salina, NY Sale sqft	Jun. 30, 2012 Santa Fe, NM Sale sqft	Jul. 31, 2012 Windsor, CT Acquisition SIR sqft	Jul. 31, 2012 Topeka, KS Acquisition SIR sqft	Aug. 31, 2012 Huntsville, AL Acquisition SIR sqft	Sep. 30, 2012 Carlsbad, CA Acquisition SIR sqft	Sep. 30, 2012 Foxborough, MA Sale sqft	Sep. 30, 2012 Chelmsford, MA Acquisition SIR sqft
Real estate properties information
Number of properties acquired or agreed to be acquired											15				5		4							2
Property square feet			76,832,000	76,832,000	72,287,000			5,975,699			5,975,699	4,917,441	2,382,462	2,534,979	506,592	21,400,000	1,063,364	1,009,940	868,395	170,052	405,699	140,162	334,075	1,058,258	12,934	76,978	268,328	143,934	1,370,974	95,000	208,850	110,882
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs											$ 817,257	$ 621,757	$ 361,100	$ 260,657	$ 132,400		$ 136,500	$ 150,600	$ 101,500	$ 49,000	$ 85,500	$ 18,900	$ 60,000	$ 195,500			$ 27,175	$ 19,400	$ 72,782	$ 24,700		$ 12,200
Assumption of mortgage debt				243,212	321,235			40,328	29,012	147,872	359,212	243,212	217,212	26,000				147,872		29,012			40,328	116,000						18,500		7,500
Real estate purchase price allocation, land												80,891	55,891	25,000			22,614	30,400	15,930	7,900	6,700	3,230	1,661				4,250	1,300	4,030	3,450		2,040
Real estate purchase price allocation, buildings and improvements												488,018	267,720	220,298			64,104	115,817	60,312	38,533	78,800	11,801	53,058				16,695	15,918	68,752	19,800		8,532
Real estate purchase price allocation, acquired real estate leases												81,346	63,839	17,507				22,189	25,542	4,733		3,869	11,375				6,230	2,182		2,934		2,292
Real estate purchase price allocation, assumed real estate lease obligations												9,811	9,594	217				5,348	284	30			3,932									217
Real estate purchase price allocation, discount (premium) on mortgage debt												18,687	16,756	1,931				12,458		2,136			2,162							1,484		447
Funded real estate improvements				80,168
Real estate purchase price, reallocation																	8,371
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs																									575	1,250					9,900
Mortgage financing related to office property sold	1,000	419																							419	1,000
Interest rate on real estate mortgage receivable (as a percent)	5.00%	6.00%																							6.00%	5.00%
Gain (loss) on sale of properties			$ 1,689	$ 2,039																					$ 158	$ 192					$ 1,689
Number of properties reclassified to continued operations						7	20