Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended							3 Months Ended		1 Months Ended		1 Months Ended	0 Months Ended				1 Months Ended
	Jan. 31, 2013 property sqft	Mar. 31, 2013 sqft	Dec. 31, 2012 sqft	Mar. 31, 2012 sqft	Mar. 31, 2013 Office property	Dec. 31, 2012 Office property	Mar. 31, 2013 Industrial property	Dec. 31, 2012 Industrial property	Mar. 31, 2013 Acquisition property sqft	Mar. 31, 2013 Acquisition SIR property sqft	Jan. 31, 2013 Sale property sqft	Feb. 16, 2012 Sale SIR sqft	Apr. 30, 2013 Sale Industrial sqft	May 08, 2013 Agreement to sell property sqft	Jan. 31, 2013 Addison, TX Acquisition SIR property sqft	Mar. 31, 2013 San Antonio, TX Acquisition SIR property sqft	Feb. 28, 2013 Provo, UT Acquisition SIR property sqft	Mar. 31, 2013 Boston, MA Sale
Real estate properties information
Number of properties acquired or agreed to be acquired									5	5					2	1	2
Property square feet		72,690,000		67,152,000					779,010	779,010	1,060,026	21,400,000	618,000	1,222,642	553,799	99,986	125,225
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs									$ 158,320	$ 158,320					$ 105,000	$ 18,600	$ 34,720
Real estate purchase price allocation, land										16,722					10,125	3,197	3,400
Real estate purchase price allocation, buildings and improvements										132,988					94,875	12,175	25,938
Real estate purchase price allocation, acquired real estate leases										8,889						3,507	5,382
Real estate purchase price allocation, assumed real estate lease obligations										279						279
Acquisition costs															188
Real estate improvements		18,130
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs											10,250		830	39,475				1,806
Mortgage financing related to office property sold	7,688										7,688
Interest rate on real estate mortgage receivable (as a percent)	6.00%										6.00%
Gain (loss) on sale of properties		$ 1,596									$ 1,260							$ 1,596
Number of properties sold or agreed to be sold	18										18			5
Number of properties held for sale					35	37	41	57
Property square feet, held for sale		5,613,825	6,673,851