Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		12 Months Ended			1 Months Ended					1 Months Ended				12 Months Ended				1 Months Ended		1 Months Ended		1 Months Ended		3 Months Ended	1 Months Ended														1 Months Ended		1 Months Ended			1 Months Ended
	Jun. 30, 2012	Apr. 30, 2012	Dec. 31, 2012 sqft	Dec. 31, 2011 property sqft	Dec. 31, 2010 sqft	Jan. 31, 2013 SIR property sqft	Dec. 31, 2012 SIR sqft	Dec. 31, 2012 Office property	Dec. 31, 2011 Office property	Dec. 31, 2012 Industrial property	Oct. 31, 2012 Acquisition	Sep. 30, 2012 Acquisition	May 31, 2012 Acquisition	Jan. 31, 2012 Acquisition	Dec. 31, 2012 Acquisition property sqft	Dec. 31, 2012 Acquisition CWH and its wholly owned subsidiaries, including SIR sqft	Dec. 31, 2012 Acquisition CWH and its wholly owned subsidiaries, but excluding SIR sqft	Dec. 31, 2012 Acquisition SIR sqft	Feb. 28, 2013 Pending acquisitions SIR property	Feb. 21, 2013 Pending acquisitions SIR sqft	Jan. 31, 2013 Sale property sqft	Feb. 29, 2012 Sale SIR sqft	Feb. 28, 2013 Agreement to sell property sqft	Mar. 31, 2011 Phoenix, AZ Acquisition	Mar. 31, 2012 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 CWH and its wholly owned subsidiaries, but excluding SIR 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	Dec. 31, 2012 Huntsville, AL Acquisition SIR	Dec. 31, 2012 Carlsbad, CA SIR	Sep. 30, 2012 Carlsbad, CA Acquisition SIR sqft	Dec. 31, 2012 Carlsbad, CA Acquisition SIR	Sep. 30, 2012 Foxborough, MA Sale sqft	Sep. 30, 2012 Chelmsford, MA Acquisition SIR sqft	Dec. 31, 2012 Chelmsford, MA Acquisition SIR	Nov. 30, 2012 Sunnyvale, CA Acquisition SIR sqft	Nov. 30, 2012 Oahu, HI Acquisition SIR sqft	Nov. 30, 2012 Sterling, VA Acquisition SIR sqft	Dec. 31, 2012 Ann Arbor, MI Acquisition SIR sqft	Dec. 31, 2012 Columbia, MD Acquisition SIR sqft
Real estate properties information
Number of properties acquired or agreed to be acquired				22		2									22				3						4
Property square feet			71,901,000	65,311,000	58,736,000	553,799	6,819,000								6,660,563	6,660,563	3,440,574	3,219,989		225,211	1,060,026	21,400,000	675,250		1,063,364	1,009,940	868,395	170,052	405,699	140,162	334,075	1,058,112	12,934	76,978	268,328	143,934	1,370,974			95,000		208,850	110,882		96,415	49,452	337,228	82,003	119,912
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs				$ 1,132,827		$ 105,000									$ 992,215	$ 992,215	$ 554,202	$ 438,013	$ 53,320					$ 136,500		$ 150,600	$ 101,500	$ 49,000	$ 85,500	$ 18,900	$ 60,000	$ 193,102			$ 27,175	$ 19,400	$ 72,782			$ 24,700			$ 12,200		$ 28,050	$ 6,300	$ 85,600	$ 16,906	$ 40,500
Real estate purchase price allocation, land																128,007	67,618	60,389						22,614		32,457	15,930	7,900	6,700	3,230	1,660	9,671			4,250	1,300	5,628			3,381			2,009		11,552	5,888	9,874	2,877	3,700
Real estate purchase price allocation, buildings and improvements																756,121	432,421	323,700						64,104		122,433	60,312	38,533	78,800	11,801	53,059	158,084			16,695	15,918	67,154			17,918			6,727		12,461	315	62,238	9,081	24,592
Real estate purchase price allocation, acquired real estate leases																156,650	99,668	56,982								20,932	25,542	4,733		3,869	11,375	37,086			6,230	2,182				4,885			3,911		4,037	97	14,615	4,948	12,208
Real estate purchase price allocation, assumed real estate lease obligations																11,564	10,437	1,127								5,710	284	30			3,932	481															1,127
Assumption of mortgage debt			359,213	321,235							116,000	40,328	29,012	147,872	359,212	359,212	333,212	26,000								147,872		29,012			40,328	116,000								18,500			7,500
Real estate purchase price allocation, discount (premium) on mortgage debt																26,073	24,142	1,931								12,458		2,136			2,162	7,386								1,484			447
Real estate purchase price allocation, other assumed liabilities																14,341	11,559	2,782								7,054				421		4,138				1,698												663
Funded real estate improvements			134,631
Real estate purchase price, reallocation																								8,371
Real estate purchase price, reallocation buildings and improvements to land																																						1,598
Real estate purchase price, reallocation from land to acquired real estate leases																																							69		69			31
Real estate purchase price, reallocation from buildings and improvements to acquired real estate leases																																							1,882		1,882			1,805
Real estate purchase price, reallocation from assumed real estate lease obligations to acquired real estate leases																																												217
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs																					10,250		5,055										575	1,250								9,900
Mortgage financing related to office property sold	1,000	419	1,419		8,288																												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					$ 34,336																												$ 158	$ 192								$ 1,689
Number of properties reclassified to continued operations									7
Number of properties sold or agreed to be sold																					18		2
Number of properties held for sale								37		57
Property square feet, held for sale			6,673,851