Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	6 Months Ended							6 Months Ended	1 Months Ended		6 Months Ended		0 Months Ended		1 Months Ended		0 Months Ended		0 Months Ended			1 Months Ended
	Jan. 31, 2013 property	Jun. 30, 2013 sqft	Aug. 06, 2013 property	Jun. 30, 2012 sqft	Jun. 30, 2013 Office property	Dec. 31, 2012 Office property	Jun. 30, 2013 Industrial property	Dec. 31, 2012 Industrial property	Jun. 30, 2013 Acquisition SIR property sqft	May 31, 2013 Sale sqft	Jan. 31, 2013 Sale property sqft	Jun. 30, 2013 Sale property sqft	Feb. 16, 2012 Sale SIR sqft	Jun. 28, 2013 Sale Office sqft	Jun. 29, 2013 Sale Office sqft	Jun. 30, 2013 Sale Office property sqft	Apr. 30, 2013 Sale Industrial sqft	Aug. 05, 2013 Agreement to sell property sqft	Jun. 30, 2013 Agreement to sell property	Jan. 31, 2013 Addison, TX Acquisition SIR property sqft	Feb. 28, 2013 Provo, UT Acquisition SIR property sqft	Mar. 31, 2013 San Antonio, TX Acquisition SIR property sqft	Mar. 31, 2013 Boston, MA Sale	Jul. 31, 2013 Richmond, VA Acquisition Office SIR sqft	Jun. 30, 2013 Tukwila, WA Sale
Real estate properties information
Number of properties acquired or agreed to be acquired									5											2	2	1
Property square feet		72,717,000		67,870,000					779,010	57,250	1,060,026	2,265,228	21,400,000	143,802	30,105	356,045	618,000	2,283,345		553,799	125,225	99,986		310,950
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs									$ 158,320											$ 105,000	$ 34,720	$ 18,600		$ 143,600
Real estate purchase price allocation, land									16,704											10,107	3,400	3,197
Real estate purchase price allocation, buildings and improvements									133,006											94,893	25,938	12,175
Real estate purchase price allocation, acquired real estate leases									8,889												5,382	3,507
Real estate purchase price allocation, acquired real estate lease obligations									(279)													(279)
Acquisition costs																				226
Real estate improvements		47,356
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs										4,025	10,250	42,612		5,250	1,600	16,300	830	67,500					1,806		2,551
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									$ 1,277				$ 317								$ 1,596		$ 1,765
Number of properties sold or agreed to be sold	18										18	24				2		49
Number of land parcels sold												2
Number of properties held for sale			49		30	37	40	57											21
Property square feet, held for sale		4,408,623